diff --git a/src/app/controller.ts b/src/app/controller.ts
new file mode 100644
index 0000000..1112600
--- /dev/null
+++ b/src/app/controller.ts
@@ -0,0 +1,134 @@
+import { songById, songs, type LandingId, type PlaceId } from "../data/album";
+import type { Store } from "./state";
+import { transitionFor } from "./voyage";
+
+/** The parts of the 3D world the controller drives (src/app/world/world.ts implements them). */
+export interface ControllerWorld {
+  sailTo(songId: string, opts?: { instant?: boolean }): Promise<void>;
+  setLanding(id: LandingId | null): Promise<void>;
+  setRibbons(motifIds: string[] | null, highlight: string | null): void;
+}
+/** The parts of the frieze curtain the controller drives (src/app/frieze/curtain.ts). */
+export interface ControllerCurtain {
+  drop(sagaId: PlaceId, opts?: { landing?: LandingId }): Promise<void>;
+  lift(): Promise<void>;
+  fade(): Promise<void>;
+}
+
+export interface Controller {
+  /** The song the stage has finished moving to. */
+  settledSongId(): string;
+  busy(): boolean;
+  play(): void;
+  stop(): void;
+  playing(): boolean;
+  onPlayingChange(fn: (playing: boolean) => void): void;
+}
+
+const PLAY_HOLD_MS = 3400;
+
+/**
+ * Moves the stage when the song changes. Steps run one at a time; a newer request makes the running step
+ * stop at its next await, and the last request wins. A curtain left down by a cancelled step is lifted by
+ * the next one, so rapid navigation never leaves the stage covered.
+ */
+export function createController(opts: {
+  store: Store;
+  world: ControllerWorld | null;
+  curtain: ControllerCurtain;
+  reducedMotion: boolean;
+}): Controller {
+  const { store, world, curtain, reducedMotion } = opts;
+  let settled = store.get().songId;
+  let landed: LandingId | null = null;
+  let curtainDown = false;
+  let gen = 0;
+  let running = 0;
+  let queue: Promise<void> = Promise.resolve();
+  let timer: ReturnType<typeof setTimeout> | null = null;
+  const playListeners: ((p: boolean) => void)[] = [];
+
+  const ribbons = () => {
+    const s = store.get();
+    world?.setRibbons(s.ribbons ? songById(s.songId)!.motifIds : null, s.motif);
+  };
+
+  async function step(to: string, my: number) {
+    if (my !== gen) return;
+    const { transition, landing } = transitionFor(settled, to);
+    const kind = reducedMotion && transition.kind !== "sail" ? "fade" : transition.kind;
+    if (kind === "curtain" && transition.kind === "curtain") {
+      await curtain.drop(transition.sagaId, landing ? { landing } : undefined);
+      curtainDown = true;
+    } else if (kind === "fade") {
+      await curtain.fade();
+      curtainDown = true;
+    }
+    if (my !== gen) return;
+    if (world) {
+      if (landed && landed !== landing) {
+        await world.setLanding(null);
+        landed = null;
+      }
+      await world.sailTo(to, { instant: kind !== "sail" });
+    }
+    if (my !== gen) return;
+    if (curtainDown) {
+      await curtain.lift();
+      curtainDown = false;
+    }
+    if (world && landing !== landed) {
+      await world.setLanding(landing);
+      landed = landing;
+    }
+    settled = to;
+    ribbons();
+    if (timer === null && playingNow) schedule();
+  }
+
+  function request(to: string) {
+    const my = ++gen;
+    running++;
+    queue = queue.then(() => step(to, my)).catch((e) => console.error(e)).finally(() => { running--; });
+  }
+
+  let playingNow = false;
+  function schedule() {
+    timer = setTimeout(() => {
+      timer = null;
+      if (!playingNow) return;
+      const next = songs.find((x) => x.track === songById(settled)!.track + 1);
+      if (!next) return setPlaying(false);
+      store.set({ songId: next.id });
+    }, PLAY_HOLD_MS);
+  }
+  function setPlaying(p: boolean) {
+    playingNow = p;
+    if (!p && timer) { clearTimeout(timer); timer = null; }
+    playListeners.forEach((fn) => fn(p));
+  }
+
+  store.subscribe((s, prev) => {
+    if (s.songId !== prev.songId) request(s.songId);
+    else if (s.ribbons !== prev.ribbons || s.motif !== prev.motif) ribbons();
+  });
+
+  // First paint: put the ship at the deep-linked song without a transition.
+  if (world) {
+    const first = songById(settled)!;
+    running++;
+    queue = world.sailTo(settled, { instant: true })
+      .then(async () => { if (first.landing) { await world.setLanding(first.landing); landed = first.landing; } ribbons(); })
+      .catch((e) => console.error(e))
+      .finally(() => { running--; });
+  }
+
+  return {
+    settledSongId: () => settled,
+    busy: () => running > 0,
+    play() { if (!playingNow) { setPlaying(true); if (!running) schedule(); } },
+    stop() { setPlaying(false); },
+    playing: () => playingNow,
+    onPlayingChange(fn) { playListeners.push(fn); },
+  };
+}
diff --git a/src/app/voyage.ts b/src/app/voyage.ts
new file mode 100644
index 0000000..b846b69
--- /dev/null
+++ b/src/app/voyage.ts
@@ -0,0 +1,18 @@
+import { sagaOf, sagas, songById, type LandingId, type PlaceId } from "../data/album";
+
+export type Transition = { kind: "sail" } | { kind: "curtain"; sagaId: PlaceId } | { kind: "fade" };
+export interface Step { transition: Transition; landing: LandingId | null }
+
+/**
+ * How the stage moves from one song to another (spec 002, goals 4–5): within a saga the ship sails; into
+ * the next or previous saga that saga's frieze drops like a curtain; across two or more sagas the stage
+ * fades through the curtain colour. The step ends in the target song's landing, if it has one.
+ */
+export function transitionFor(fromSongId: string, toSongId: string): Step {
+  const from = songById(fromSongId)!;
+  const to = songById(toSongId)!;
+  const order = (id: PlaceId) => sagas.findIndex((s) => s.id === id);
+  const gap = Math.abs(order(sagaOf(to).id) - order(sagaOf(from).id));
+  const transition: Transition = gap === 0 ? { kind: "sail" } : gap === 1 ? { kind: "curtain", sagaId: to.sagaId } : { kind: "fade" };
+  return { transition, landing: to.landing };
+}
diff --git a/tests/voyage.test.ts b/tests/voyage.test.ts
new file mode 100644
index 0000000..115e563
--- /dev/null
+++ b/tests/voyage.test.ts
@@ -0,0 +1,82 @@
+import { expect, test } from "bun:test";
+import { transitionFor } from "../src/app/voyage";
+import { createController, type ControllerWorld, type ControllerCurtain } from "../src/app/controller";
+import { createStore, DEFAULT_STATE } from "../src/app/state";
+
+test("within a saga the ship sails, and landing songs land", () => {
+  expect(transitionFor("polyphemus", "survive")).toEqual({ transition: { kind: "sail" }, landing: "cave" });
+  expect(transitionFor("survive", "remember-them")).toEqual({ transition: { kind: "sail" }, landing: null });
+});
+
+test("into the next or previous saga the curtain drops with that saga's frieze", () => {
+  expect(transitionFor("warrior-of-the-mind", "polyphemus")).toEqual({ transition: { kind: "curtain", sagaId: "cyclops" }, landing: "cave" });
+  expect(transitionFor("polyphemus", "warrior-of-the-mind")).toEqual({ transition: { kind: "curtain", sagaId: "troy" }, landing: null });
+});
+
+test("jumps across two or more sagas fade", () => {
+  expect(transitionFor("storm", "god-games")).toEqual({ transition: { kind: "fade" }, landing: "arena" });
+});
+
+test("the same song is a no-op sail", () => {
+  expect(transitionFor("storm", "storm")).toEqual({ transition: { kind: "sail" }, landing: null });
+});
+
+function fakes() {
+  const log: string[] = [];
+  let release: (() => void)[] = [];
+  const later = (what: string) => new Promise<void>((r) => { log.push(what); release.push(r); });
+  const world: ControllerWorld = {
+    sailTo: (id) => later(`sail:${id}`),
+    setLanding: (id) => later(`land:${id}`),
+    setRibbons: () => {},
+  };
+  const curtain: ControllerCurtain = {
+    drop: (saga) => later(`drop:${saga}`),
+    lift: () => later("lift"),
+    fade: () => later("fade"),
+  };
+  const flush = async () => {
+    for (let i = 0; i < 50; i++) {
+      const r = release; release = [];
+      r.forEach((f) => f());
+      await Promise.resolve(); await Promise.resolve();
+    }
+  };
+  return { log, world, curtain, flush };
+}
+
+test("rapid navigation ends at the last requested song with the curtain lifted", async () => {
+  const f = fakes();
+  const store = createStore(DEFAULT_STATE, { history: false });
+  const ctl = createController({ store, world: f.world, curtain: f.curtain, reducedMotion: false });
+  for (const id of ["just-a-man", "polyphemus", "storm", "puppeteer", "god-games"]) store.set({ songId: id });
+  await f.flush();
+  expect(ctl.settledSongId()).toBe("god-games");
+  const lastSail = f.log.filter((l) => l.startsWith("sail:")).at(-1);
+  expect(lastSail).toBe("sail:god-games");
+  const drops = f.log.filter((l) => l.startsWith("drop:") || l === "fade").length;
+  const lifts = f.log.filter((l) => l === "lift").length;
+  expect(lifts).toBeGreaterThanOrEqual(drops === 0 ? 0 : 1);
+  expect(f.log.at(-1)).toBe("land:arena");
+  expect(ctl.busy()).toBe(false);
+});
+
+test("without a world the controller still follows the songs (frieze-only browsing)", async () => {
+  const f = fakes();
+  const store = createStore(DEFAULT_STATE, { history: false });
+  const ctl = createController({ store, world: null, curtain: f.curtain, reducedMotion: false });
+  store.set({ songId: "polyphemus" });
+  await f.flush();
+  expect(ctl.settledSongId()).toBe("polyphemus");
+  expect(f.log).toContain("drop:cyclops");
+});
+
+test("reduced motion turns every scene change into a fade", async () => {
+  const f = fakes();
+  const store = createStore(DEFAULT_STATE, { history: false });
+  createController({ store, world: f.world, curtain: f.curtain, reducedMotion: true });
+  store.set({ songId: "polyphemus" });
+  await f.flush();
+  expect(f.log).toContain("fade");
+  expect(f.log.some((l) => l.startsWith("drop:"))).toBe(false);
+});