diff --git a/apps/api/src/combat/combat.service.spec.ts b/apps/api/src/combat/combat.service.spec.ts
new file mode 100644
index 0000000..539a5ff
--- /dev/null
+++ b/apps/api/src/combat/combat.service.spec.ts
@@ -0,0 +1,569 @@
+import { DataSource, EntityManager, EntityTarget } from 'typeorm';
+import { CharacterCombatStatsService } from '../characters/character-combat-stats.service';
+import { Character } from '../characters/entities/character.entity';
+import { Hunt } from '../hunting/entities/hunt.entity';
+import { HuntEncounter } from '../hunting/entities/hunt-encounter.entity';
+import { HuntStatus } from '../hunting/hunt-status.enum';
+import { MonsterDefinition } from '../monsters/entities/monster-definition.entity';
+import { TravelService } from '../travel/travel.service';
+import { CombatAction } from './combat-action.enum';
+import { CombatEngineService } from './combat-engine.service';
+import { CombatDomainError } from './combat.errors';
+import { CombatService } from './combat.service';
+import { CombatStatus } from './combat-status.enum';
+import { CombatEvent } from './entities/combat-event.entity';
+import { Combat } from './entities/combat.entity';
+
+const CHARACTER_ID = '10000000-0000-4000-8000-000000000001';
+const OTHER_CHARACTER_ID = '10000000-0000-4000-8000-000000000002';
+const HUNT_ID = '20000000-0000-4000-8000-000000000001';
+const ENCOUNTER_ID = '30000000-0000-4000-8000-000000000001';
+const MONSTER_ID = '40000000-0000-4000-8000-000000000001';
+
+interface FakeState {
+  characters: Character[];
+  hunts: Hunt[];
+  huntEncounters: HuntEncounter[];
+  monsters: MonsterDefinition[];
+  combats: Combat[];
+  combatEvents: CombatEvent[];
+}
+
+class FakeRepository<T extends { id: string }> {
+  constructor(
+    private readonly state: FakeState,
+    private readonly target: EntityTarget<T>,
+    private readonly inTransaction: boolean,
+    private readonly dataSource: FakeDataSource,
+  ) {}
+
+  findOne(options: {
+    where: Partial<T>;
+    lock?: { mode: string };
+  }): Promise<T | null> {
+    if (options.lock) {
+      if (!this.inTransaction) {
+        throw new Error('Pessimistic locks require a transaction');
+      }
+      this.dataSource.locks.push({
+        target: this.target,
+        mode: options.lock.mode,
+      });
+    }
+    return Promise.resolve(
+      this.rows().find((row) => this.matches(row, options.where)) ?? null,
+    );
+  }
+
+  findOneBy(where: Partial<T>): Promise<T | null> {
+    return Promise.resolve(
+      this.rows().find((row) => this.matches(row, where)) ?? null,
+    );
+  }
+
+  find(options: {
+    where: Partial<T>;
+    order?: Partial<Record<keyof T, 'ASC' | 'DESC'>>;
+  }): Promise<T[]> {
+    const matched = this.rows().filter((row) =>
+      this.matches(row, options.where),
+    );
+    const orderKey = options.order
+      ? (Object.keys(options.order)[0] as keyof T)
+      : undefined;
+    if (orderKey) {
+      const direction = options.order![orderKey] === 'DESC' ? -1 : 1;
+      matched.sort((a, b) => {
+        if (a[orderKey] === b[orderKey]) return 0;
+        return a[orderKey] > b[orderKey] ? direction : -direction;
+      });
+    }
+    return Promise.resolve(matched);
+  }
+
+  count(options: { where: Partial<T> }): Promise<number> {
+    return Promise.resolve(
+      this.rows().filter((row) => this.matches(row, options.where)).length,
+    );
+  }
+
+  create(values: Partial<T>): T {
+    return { ...values } as T;
+  }
+
+  save(entity: T): Promise<T> {
+    if (!entity.id) {
+      entity.id = this.dataSource.nextId(this.targetName());
+    }
+    const rows = this.rows();
+    const index = rows.findIndex((row) => row.id === entity.id);
+    if (index === -1) {
+      rows.push(entity);
+    } else {
+      rows[index] = entity;
+    }
+    return Promise.resolve(entity);
+  }
+
+  private rows(): T[] {
+    if (this.target === Character) return this.state.characters as T[];
+    if (this.target === Hunt) return this.state.hunts as T[];
+    if (this.target === HuntEncounter) return this.state.huntEncounters as T[];
+    if (this.target === MonsterDefinition) return this.state.monsters as T[];
+    if (this.target === Combat) return this.state.combats as T[];
+    if (this.target === CombatEvent) return this.state.combatEvents as T[];
+    throw new Error(`Unsupported repository ${this.targetName()}`);
+  }
+
+  private matches(row: T, where: Partial<T>): boolean {
+    return Object.entries(where).every(
+      ([key, value]) => row[key as keyof T] === value,
+    );
+  }
+
+  private targetName(): string {
+    return typeof this.target === 'function'
+      ? this.target.name
+      : 'EntitySchema';
+  }
+}
+
+class FakeEntityManager {
+  constructor(
+    private readonly state: FakeState,
+    private readonly dataSource: FakeDataSource,
+  ) {}
+
+  getRepository<T extends { id: string }>(target: EntityTarget<T>) {
+    return new FakeRepository(this.state, target, true, this.dataSource);
+  }
+}
+
+class FakeDataSource {
+  readonly locks: Array<{ target: EntityTarget<unknown>; mode: string }> = [];
+  private readonly idCounters = new Map<string, number>();
+
+  constructor(public state: FakeState) {}
+
+  getRepository<T extends { id: string }>(target: EntityTarget<T>) {
+    return new FakeRepository(this.state, target, false, this);
+  }
+
+  async transaction<T>(
+    work: (manager: EntityManager) => Promise<T>,
+  ): Promise<T> {
+    const draft = structuredClone(this.state);
+    const result = await work(
+      new FakeEntityManager(draft, this) as unknown as EntityManager,
+    );
+    this.state = draft;
+    return result;
+  }
+
+  nextId(targetName: string): string {
+    const next = (this.idCounters.get(targetName) ?? 0) + 1;
+    this.idCounters.set(targetName, next);
+    return `${targetName.toLowerCase()}-generated-${next}`;
+  }
+}
+
+function character(overrides: Partial<Character> = {}): Character {
+  return {
+    id: CHARACTER_ID,
+    name: 'Aric Duskwalker',
+    level: 1,
+    experience: 0,
+    baseHp: 100,
+    baseAttack: 6,
+    currentHp: 100,
+    currentLocationId: 'location-1',
+    createdAt: new Date('2026-08-18T09:00:00.000Z'),
+    updatedAt: new Date('2026-08-18T09:00:00.000Z'),
+    ...overrides,
+  } as Character;
+}
+
+function monster(
+  overrides: Partial<MonsterDefinition> = {},
+): MonsterDefinition {
+  return {
+    id: MONSTER_ID,
+    key: 'ash-rat',
+    name: 'Aschenratte',
+    level: 1,
+    maxHp: 45,
+    attack: 5,
+    armor: 0,
+    experienceReward: 8,
+    silverMin: 4,
+    silverMax: 7,
+    artworkPath: '/images/monsters/ash-rat.png',
+    createdAt: new Date('2026-08-18T09:00:00.000Z'),
+    updatedAt: new Date('2026-08-18T09:00:00.000Z'),
+    ...overrides,
+  };
+}
+
+function hunt(overrides: Partial<Hunt> = {}): Hunt {
+  return {
+    id: HUNT_ID,
+    characterId: CHARACTER_ID,
+    locationId: 'location-1',
+    status: HuntStatus.ACTIVE,
+    createdAt: new Date('2026-08-18T09:00:00.000Z'),
+    ...overrides,
+  } as Hunt;
+}
+
+function huntEncounter(overrides: Partial<HuntEncounter> = {}): HuntEncounter {
+  return {
+    id: ENCOUNTER_ID,
+    huntId: HUNT_ID,
+    monsterDefinitionId: MONSTER_ID,
+    position: 0,
+    consumedAt: null,
+    createdAt: new Date('2026-08-18T09:00:00.000Z'),
+    ...overrides,
+  } as HuntEncounter;
+}
+
+function createState(overrides: Partial<FakeState> = {}): FakeState {
+  return {
+    characters: [character()],
+    hunts: [hunt()],
+    huntEncounters: [huntEncounter()],
+    monsters: [monster()],
+    combats: [],
+    combatEvents: [],
+    ...overrides,
+  };
+}
+
+function fakeTravelService(
+  status: 'IDLE' | 'TRAVELLING' = 'IDLE',
+): TravelService {
+  return {
+    completeTravelIfDue: jest.fn().mockResolvedValue({ status }),
+  } as unknown as TravelService;
+}
+
+function createService(
+  options: { state?: FakeState; travelService?: TravelService } = {},
+) {
+  const state = options.state ?? createState();
+  const dataSource = new FakeDataSource(state);
+  const travelService = options.travelService ?? fakeTravelService();
+  const combatEngine = new CombatEngineService();
+  const characterCombatStats = new CharacterCombatStatsService();
+  const service = new CombatService(
+    dataSource as unknown as DataSource,
+    travelService,
+    combatEngine,
+    characterCombatStats,
+  );
+  return { dataSource, service, travelService };
+}
+
+async function expectCombatDomainError(
+  promise: Promise<unknown>,
+  code: string,
+): Promise<void> {
+  let error: unknown;
+  try {
+    await promise;
+  } catch (cause) {
+    error = cause;
+  }
+  expect(error).toBeInstanceOf(CombatDomainError);
+  if (!(error instanceof CombatDomainError)) {
+    throw new Error('Expected CombatDomainError');
+  }
+  expect(error.code).toBe(code);
+}
+
+describe('CombatService', () => {
+  describe('startCombat', () => {
+    it('starts an ACTIVE combat with snapshotted stats and full HP', async () => {
+      const { dataSource, service } = createService();
+
+      const combat = await service.startCombat(CHARACTER_ID, ENCOUNTER_ID);
+
+      expect(combat.status).toBe('ACTIVE');
+      expect(combat.round).toBe(1);
+      expect(combat.player).toEqual({
+        name: 'Aric Duskwalker',
+        maxHp: 100,
+        currentHp: 100,
+      });
+      expect(combat.monster).toEqual({
+        key: 'ash-rat',
+        name: 'Aschenratte',
+        level: 1,
+        maxHp: 45,
+        currentHp: 45,
+        artworkPath: '/images/monsters/ash-rat.png',
+      });
+      expect(combat.events).toEqual([]);
+      expect(dataSource.state.combats).toHaveLength(1);
+      expect(dataSource.state.combats[0]).toMatchObject({
+        characterId: CHARACTER_ID,
+        huntEncounterId: ENCOUNTER_ID,
+        monsterDefinitionId: MONSTER_ID,
+        status: CombatStatus.ACTIVE,
+        playerState: { attack: 6, weaponDamage: 8, armor: 6 },
+        monsterState: { attack: 5, armor: 0 },
+      });
+    });
+
+    it('marks the encounter as consumed', async () => {
+      const { dataSource, service } = createService();
+
+      await service.startCombat(CHARACTER_ID, ENCOUNTER_ID);
+
+      expect(dataSource.state.huntEncounters[0].consumedAt).not.toBeNull();
+    });
+
+    it('rejects an unknown encounter id', async () => {
+      const { service } = createService();
+
+      await expectCombatDomainError(
+        service.startCombat(CHARACTER_ID, 'unknown-id'),
+        'HUNT_ENCOUNTER_NOT_FOUND',
+      );
+    });
+
+    it('rejects an already-consumed encounter, and does not create a second combat', async () => {
+      const state = createState({
+        huntEncounters: [
+          huntEncounter({ consumedAt: new Date('2026-08-18T09:05:00.000Z') }),
+        ],
+      });
+      const { dataSource, service } = createService({ state });
+
+      await expectCombatDomainError(
+        service.startCombat(CHARACTER_ID, ENCOUNTER_ID),
+        'HUNT_ENCOUNTER_ALREADY_CONSUMED',
+      );
+      expect(dataSource.state.combats).toHaveLength(0);
+    });
+
+    it('rejects an encounter belonging to a different character', async () => {
+      const state = createState({
+        characters: [character(), character({ id: OTHER_CHARACTER_ID })],
+      });
+      const { service } = createService({ state });
+
+      await expectCombatDomainError(
+        service.startCombat(OTHER_CHARACTER_ID, ENCOUNTER_ID),
+        'INVALID_HUNT_ENCOUNTER',
+      );
+    });
+
+    it('rejects an encounter whose hunt is no longer ACTIVE', async () => {
+      const state = createState({
+        hunts: [hunt({ status: HuntStatus.SUPERSEDED })],
+      });
+      const { service } = createService({ state });
+
+      await expectCombatDomainError(
+        service.startCombat(CHARACTER_ID, ENCOUNTER_ID),
+        'INVALID_HUNT_ENCOUNTER',
+      );
+    });
+
+    it('rejects starting combat while the character is travelling', async () => {
+      const { service } = createService({
+        travelService: fakeTravelService('TRAVELLING'),
+      });
+
+      await expectCombatDomainError(
+        service.startCombat(CHARACTER_ID, ENCOUNTER_ID),
+        'CHARACTER_TRAVELLING',
+      );
+    });
+
+    it('rejects starting a second combat while one is already ACTIVE', async () => {
+      const state = createState({
+        combats: [
+          {
+            id: 'combat-existing',
+            characterId: CHARACTER_ID,
+            huntEncounterId: 'other-encounter',
+            monsterDefinitionId: MONSTER_ID,
+            status: CombatStatus.ACTIVE,
+            round: 1,
+            playerMaxHp: 100,
+            playerCurrentHp: 100,
+            monsterMaxHp: 45,
+            monsterCurrentHp: 45,
+            playerState: { attack: 6, weaponDamage: 8, armor: 6 },
+            monsterState: { attack: 5, armor: 0 },
+            createdAt: new Date(),
+            updatedAt: new Date(),
+            completedAt: null,
+          } as Combat,
+        ],
+      });
+      const { service } = createService({ state });
+
+      await expectCombatDomainError(
+        service.startCombat(CHARACTER_ID, ENCOUNTER_ID),
+        'COMBAT_ALREADY_ACTIVE',
+      );
+    });
+
+    it('locks the character and the active-combat lookup', async () => {
+      const { dataSource, service } = createService();
+
+      await service.startCombat(CHARACTER_ID, ENCOUNTER_ID);
+
+      expect(dataSource.locks).toEqual(
+        expect.arrayContaining([
+          { target: Character, mode: 'pessimistic_write' },
+          { target: Combat, mode: 'pessimistic_write' },
+        ]),
+      );
+    });
+  });
+
+  describe('performAction', () => {
+    async function startedCombat(state = createState()) {
+      const context = createService({ state });
+      const combat = await context.service.startCombat(
+        CHARACTER_ID,
+        ENCOUNTER_ID,
+      );
+      return { ...context, combatId: combat.id };
+    }
+
+    it('resolves ATTACK, persists HP/round changes, and returns them', async () => {
+      const { dataSource, service, combatId } = await startedCombat();
+
+      const result = await service.performAction(
+        CHARACTER_ID,
+        combatId,
+        CombatAction.ATTACK,
+      );
+
+      expect(result.status).toBe('ACTIVE');
+      expect(result.round).toBe(2);
+      expect(result.monster.currentHp).toBe(45 - 14);
+      expect(result.player.currentHp).toBe(100 - 5);
+      expect(dataSource.state.combats[0].round).toBe(2);
+      expect(dataSource.state.combats[0].monsterCurrentHp).toBe(31);
+      expect(dataSource.state.combats[0].playerCurrentHp).toBe(95);
+    });
+
+    it('persists ordered, sequential CombatEvents across multiple rounds', async () => {
+      const { dataSource, service, combatId } = await startedCombat();
+
+      await service.performAction(CHARACTER_ID, combatId, CombatAction.ATTACK);
+      await service.performAction(CHARACTER_ID, combatId, CombatAction.ATTACK);
+
+      const events = dataSource.state.combatEvents
+        .filter((event) => event.combatId === combatId)
+        .sort((a, b) => a.sequence - b.sequence);
+      expect(events.map((event) => event.sequence)).toEqual([1, 2, 3, 4]);
+      expect(events.map((event) => event.round)).toEqual([1, 1, 2, 2]);
+      expect(events[0]).toMatchObject({
+        source: 'PLAYER',
+        target: 'MONSTER',
+        type: 'DAMAGE',
+        amount: 14,
+      });
+      expect(events[1]).toMatchObject({
+        source: 'MONSTER',
+        target: 'PLAYER',
+        type: 'DAMAGE',
+        amount: 5,
+      });
+    });
+
+    it('ends the combat as WON, stops persisting new rounds, and rejects further actions', async () => {
+      const state = createState({ monsters: [monster({ maxHp: 10 })] });
+      const { dataSource, service, combatId } = await startedCombat(state);
+
+      const result = await service.performAction(
+        CHARACTER_ID,
+        combatId,
+        CombatAction.ATTACK,
+      );
+
+      expect(result.status).toBe('WON');
+      expect(dataSource.state.combats[0].completedAt).not.toBeNull();
+      await expectCombatDomainError(
+        service.performAction(CHARACTER_ID, combatId, CombatAction.ATTACK),
+        'COMBAT_ALREADY_FINISHED',
+      );
+    });
+
+    it('rejects actions on an unknown combat id', async () => {
+      const { service } = createService();
+
+      await expectCombatDomainError(
+        service.performAction(
+          CHARACTER_ID,
+          'unknown-combat',
+          CombatAction.ATTACK,
+        ),
+        'COMBAT_NOT_FOUND',
+      );
+    });
+
+    it('rejects actions from a character who does not own the combat', async () => {
+      const { service, combatId } = await startedCombat();
+
+      await expectCombatDomainError(
+        service.performAction(
+          OTHER_CHARACTER_ID,
+          combatId,
+          CombatAction.ATTACK,
+        ),
+        'COMBAT_NOT_FOUND',
+      );
+    });
+
+    it('locks the combat row for the duration of the action', async () => {
+      const { dataSource, service, combatId } = await startedCombat();
+
+      await service.performAction(CHARACTER_ID, combatId, CombatAction.ATTACK);
+
+      expect(dataSource.locks).toEqual(
+        expect.arrayContaining([{ target: Combat, mode: 'pessimistic_write' }]),
+      );
+    });
+  });
+
+  describe('getCombat', () => {
+    it('returns the persisted state and ordered events after a refresh', async () => {
+      const context = createService();
+      const started = await context.service.startCombat(
+        CHARACTER_ID,
+        ENCOUNTER_ID,
+      );
+      await context.service.performAction(
+        CHARACTER_ID,
+        started.id,
+        CombatAction.ATTACK,
+      );
+
+      const reloaded = await context.service.getCombat(
+        CHARACTER_ID,
+        started.id,
+      );
+
+      expect(reloaded.round).toBe(2);
+      expect(reloaded.monster.currentHp).toBe(31);
+      expect(reloaded.events.map((event) => event.sequence)).toEqual([1, 2]);
+    });
+
+    it('rejects an unknown combat id', async () => {
+      const { service } = createService();
+
+      await expectCombatDomainError(
+        service.getCombat(CHARACTER_ID, 'unknown'),
+        'COMBAT_NOT_FOUND',
+      );
+    });
+  });
+});
diff --git a/apps/api/src/combat/combat.service.ts b/apps/api/src/combat/combat.service.ts
new file mode 100644
index 0000000..e85434d
--- /dev/null
+++ b/apps/api/src/combat/combat.service.ts
@@ -0,0 +1,339 @@
+import { Injectable } from '@nestjs/common';
+import { DataSource, Repository } from 'typeorm';
+import { CharacterCombatStatsService } from '../characters/character-combat-stats.service';
+import { Character } from '../characters/entities/character.entity';
+import { Hunt } from '../hunting/entities/hunt.entity';
+import { HuntEncounter } from '../hunting/entities/hunt-encounter.entity';
+import { HuntStatus } from '../hunting/hunt-status.enum';
+import { MonsterDefinition } from '../monsters/entities/monster-definition.entity';
+import { TravelService } from '../travel/travel.service';
+import { TravelStatus } from '../travel/travel-status.enum';
+import { CombatAction } from './combat-action.enum';
+import { CombatEngineService } from './combat-engine.service';
+import { CombatEngineState } from './combat-engine.types';
+import {
+  characterNotFound,
+  characterTravelling,
+  combatAlreadyActive,
+  combatAlreadyFinished,
+  combatNotFound,
+  combatStateInvalid,
+  huntEncounterAlreadyConsumed,
+  huntEncounterNotFound,
+  invalidHuntEncounter,
+} from './combat.errors';
+import { CombatStatus } from './combat-status.enum';
+import { CombatEvent } from './entities/combat-event.entity';
+import { Combat } from './entities/combat.entity';
+
+export interface CombatPlayerDto {
+  name: string;
+  maxHp: number;
+  currentHp: number;
+}
+
+export interface CombatMonsterDto {
+  key: string;
+  name: string;
+  level: number;
+  maxHp: number;
+  currentHp: number;
+  artworkPath: string;
+}
+
+export interface CombatEventDto {
+  round: number;
+  sequence: number;
+  type: string;
+  source: string;
+  target: string;
+  amount?: number;
+}
+
+export interface CombatDto {
+  id: string;
+  status: CombatStatus;
+  round: number;
+  player: CombatPlayerDto;
+  monster: CombatMonsterDto;
+  events: CombatEventDto[];
+}
+
+@Injectable()
+export class CombatService {
+  constructor(
+    private readonly dataSource: DataSource,
+    private readonly travelService: TravelService,
+    private readonly combatEngine: CombatEngineService,
+    private readonly characterCombatStats: CharacterCombatStatsService,
+  ) {}
+
+  async startCombat(
+    characterId: string,
+    encounterId: string,
+  ): Promise<CombatDto> {
+    const travel = await this.travelService.completeTravelIfDue(characterId);
+    if (travel.status === TravelStatus.TRAVELLING) {
+      throw characterTravelling();
+    }
+
+    return this.dataSource.transaction(async (manager) => {
+      const characters = manager.getRepository(Character);
+      const encounters = manager.getRepository(HuntEncounter);
+      const hunts = manager.getRepository(Hunt);
+      const monsters = manager.getRepository(MonsterDefinition);
+      const combats = manager.getRepository(Combat);
+
+      const character = await this.lockCharacter(characters, characterId);
+
+      const encounter = await encounters.findOne({
+        where: { id: encounterId },
+        lock: { mode: 'pessimistic_write' },
+      });
+      if (!encounter) {
+        throw huntEncounterNotFound();
+      }
+      if (encounter.consumedAt) {
+        throw huntEncounterAlreadyConsumed();
+      }
+
+      const hunt = await hunts.findOneBy({ id: encounter.huntId });
+      if (
+        !hunt ||
+        hunt.characterId !== characterId ||
+        hunt.status !== HuntStatus.ACTIVE
+      ) {
+        throw invalidHuntEncounter();
+      }
+
+      const existingActiveCombat = await combats.findOne({
+        where: { characterId, status: CombatStatus.ACTIVE },
+        lock: { mode: 'pessimistic_write' },
+      });
+      if (existingActiveCombat) {
+        throw combatAlreadyActive();
+      }
+
+      const monster = await monsters.findOneBy({
+        id: encounter.monsterDefinitionId,
+      });
+      if (!monster) {
+        throw invalidHuntEncounter();
+      }
+
+      const playerStats = this.characterCombatStats.getStats(character);
+
+      const combat = combats.create({
+        characterId,
+        huntEncounterId: encounter.id,
+        monsterDefinitionId: monster.id,
+        status: CombatStatus.ACTIVE,
+        round: 1,
+        playerMaxHp: playerStats.maxHp,
+        playerCurrentHp: playerStats.maxHp,
+        monsterMaxHp: monster.maxHp,
+        monsterCurrentHp: monster.maxHp,
+        playerState: {
+          attack: playerStats.attack,
+          weaponDamage: playerStats.weaponDamage,
+          armor: playerStats.armor,
+        },
+        monsterState: { attack: monster.attack, armor: monster.armor },
+        completedAt: null,
+      });
+      await combats.save(combat);
+
+      encounter.consumedAt = new Date();
+      await encounters.save(encounter);
+
+      return this.toCombatDto(combat, character.name, monster, []);
+    });
+  }
+
+  async getCombat(characterId: string, combatId: string): Promise<CombatDto> {
+    const combats = this.dataSource.getRepository(Combat);
+    const combat = await combats.findOne({
+      where: { id: combatId, characterId },
+    });
+    if (!combat) {
+      throw combatNotFound();
+    }
+
+    const [character, monster, events] = await Promise.all([
+      this.loadCharacter(combat.characterId),
+      this.loadMonster(combat.monsterDefinitionId),
+      this.loadEvents(combat.id),
+    ]);
+
+    return this.toCombatDto(combat, character.name, monster, events);
+  }
+
+  async performAction(
+    characterId: string,
+    combatId: string,
+    action: CombatAction,
+  ): Promise<CombatDto> {
+    return this.dataSource.transaction(async (manager) => {
+      const combats = manager.getRepository(Combat);
+      const combatEvents = manager.getRepository(CombatEvent);
+
+      const combat = await combats.findOne({
+        where: { id: combatId, characterId },
+        lock: { mode: 'pessimistic_write' },
+      });
+      if (!combat) {
+        throw combatNotFound();
+      }
+      if (combat.status !== CombatStatus.ACTIVE) {
+        throw combatAlreadyFinished();
+      }
+
+      const actionRound = combat.round;
+      const engineState = this.toEngineState(combat);
+      const result = this.combatEngine.resolveAction(engineState, { action });
+
+      combat.round = result.state.round;
+      combat.status = result.state.status;
+      combat.playerCurrentHp = result.state.player.currentHp;
+      combat.monsterCurrentHp = result.state.monster.currentHp;
+      if (combat.status !== CombatStatus.ACTIVE) {
+        combat.completedAt = new Date();
+      }
+      await combats.save(combat);
+
+      const startingSequence = await combatEvents.count({
+        where: { combatId: combat.id },
+      });
+      for (let index = 0; index < result.events.length; index += 1) {
+        const event = result.events[index];
+        const entity = combatEvents.create({
+          combatId: combat.id,
+          round: actionRound,
+          sequence: startingSequence + index + 1,
+          type: event.type,
+          source: event.source,
+          target: event.target,
+          amount: event.amount ?? null,
+        });
+        await combatEvents.save(entity);
+      }
+
+      const [character, monster, events] = await Promise.all([
+        this.loadCharacter(
+          combat.characterId,
+          manager.getRepository(Character),
+        ),
+        this.loadMonster(
+          combat.monsterDefinitionId,
+          manager.getRepository(MonsterDefinition),
+        ),
+        this.loadEvents(combat.id, combatEvents),
+      ]);
+
+      return this.toCombatDto(combat, character.name, monster, events);
+    });
+  }
+
+  private async lockCharacter(
+    characters: Repository<Character>,
+    characterId: string,
+  ): Promise<Character> {
+    const character = await characters.findOne({
+      where: { id: characterId },
+      lock: { mode: 'pessimistic_write' },
+    });
+    if (!character) {
+      throw characterNotFound();
+    }
+    return character;
+  }
+
+  private async loadCharacter(
+    characterId: string,
+    repo?: Repository<Character>,
+  ): Promise<Character> {
+    const characters = repo ?? this.dataSource.getRepository(Character);
+    const character = await characters.findOneBy({ id: characterId });
+    if (!character) {
+      // combats.character_id is a RESTRICT FK; a persisted combat's
+      // character is guaranteed to exist.
+      throw combatStateInvalid();
+    }
+    return character;
+  }
+
+  private async loadMonster(
+    monsterId: string,
+    repo?: Repository<MonsterDefinition>,
+  ): Promise<MonsterDefinition> {
+    const monsters = repo ?? this.dataSource.getRepository(MonsterDefinition);
+    const monster = await monsters.findOneBy({ id: monsterId });
+    if (!monster) {
+      // combats.monster_definition_id is a RESTRICT FK; guaranteed to exist.
+      throw combatStateInvalid();
+    }
+    return monster;
+  }
+
+  private loadEvents(
+    combatId: string,
+    repo?: Repository<CombatEvent>,
+  ): Promise<CombatEvent[]> {
+    const combatEvents = repo ?? this.dataSource.getRepository(CombatEvent);
+    return combatEvents.find({
+      where: { combatId },
+      order: { sequence: 'ASC' },
+    });
+  }
+
+  private toEngineState(combat: Combat): CombatEngineState {
+    return {
+      status: combat.status,
+      round: combat.round,
+      player: {
+        currentHp: combat.playerCurrentHp,
+        maxHp: combat.playerMaxHp,
+        stats: combat.playerState,
+      },
+      monster: {
+        currentHp: combat.monsterCurrentHp,
+        maxHp: combat.monsterMaxHp,
+        stats: combat.monsterState,
+      },
+    };
+  }
+
+  private toCombatDto(
+    combat: Combat,
+    playerName: string,
+    monster: MonsterDefinition,
+    events: CombatEvent[],
+  ): CombatDto {
+    return {
+      id: combat.id,
+      status: combat.status,
+      round: combat.round,
+      player: {
+        name: playerName,
+        maxHp: combat.playerMaxHp,
+        currentHp: combat.playerCurrentHp,
+      },
+      monster: {
+        key: monster.key,
+        name: monster.name,
+        level: monster.level,
+        maxHp: combat.monsterMaxHp,
+        currentHp: combat.monsterCurrentHp,
+        artworkPath: monster.artworkPath,
+      },
+      events: events.map((event) => ({
+        round: event.round,
+        sequence: event.sequence,
+        type: event.type,
+        source: event.source,
+        target: event.target,
+        amount: event.amount ?? undefined,
+      })),
+    };
+  }
+}