diff --git a/src/Chip8.Replay/Timeline.cs b/src/Chip8.Replay/Timeline.cs
new file mode 100644
index 0000000..dca6582
--- /dev/null
+++ b/src/Chip8.Replay/Timeline.cs
@@ -0,0 +1,140 @@
+using System;
+using Chip8.Core;
+
+namespace Chip8.Replay;
+
+public sealed class Timeline
+{
+    public const int defaultKeyframeInterval = 8;
+    public const int defaultCapacity = 256;
+
+    public Machine machine { get; }
+    public InputMovie movie { get; }
+    public RewindBuffer rewind { get; }
+    public int keyframeInterval { get; }
+    public int currentFrame { get; private set; }
+
+    int nextKeyframeFrame;
+
+    Timeline(Machine machine, InputMovie movie, int keyframeInterval, int capacity)
+    {
+        this.machine = machine;
+        this.movie = movie;
+        this.keyframeInterval = keyframeInterval;
+        rewind = new RewindBuffer(capacity);
+        currentFrame = 0;
+        nextKeyframeFrame = 0;
+    }
+
+    public static Timeline forMovie(InputMovie movie, byte[] rom,
+        int keyframeInterval = defaultKeyframeInterval, int capacity = defaultCapacity)
+    {
+        if (keyframeInterval < 1)
+        {
+            throw new ArgumentOutOfRangeException(nameof(keyframeInterval));
+        }
+        var machine = MachineBuilder.fromMovie(movie, rom);
+        return new Timeline(machine, movie, keyframeInterval, capacity);
+    }
+
+    public static Timeline forMachine(Machine machine, InputMovie movie,
+        int keyframeInterval = defaultKeyframeInterval, int capacity = defaultCapacity)
+    {
+        if (keyframeInterval < 1)
+        {
+            throw new ArgumentOutOfRangeException(nameof(keyframeInterval));
+        }
+        return new Timeline(machine, movie, keyframeInterval, capacity);
+    }
+
+    public int instructionsPerFrame => movie.instructionsPerFrame;
+
+    public void advanceFrame()
+    {
+        maybeRecordKeyframe();
+        stepForwardOnce();
+    }
+
+    public void advanceFrames(int count)
+    {
+        for (int frame = 0; frame < count; frame++)
+        {
+            advanceFrame();
+        }
+    }
+
+    public void advanceTo(int frame)
+    {
+        if (frame < currentFrame)
+        {
+            throw new ArgumentOutOfRangeException(nameof(frame), "advanceTo cannot move backward; use rewindTo");
+        }
+        while (currentFrame < frame)
+        {
+            advanceFrame();
+        }
+    }
+
+    public void rewindTo(int frame)
+    {
+        if (frame < 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(frame));
+        }
+        if (frame > currentFrame)
+        {
+            throw new ArgumentOutOfRangeException(nameof(frame), "rewindTo cannot move forward; use advanceTo");
+        }
+        if (frame == currentFrame)
+        {
+            return;
+        }
+
+        TimelineSnapshot? keyframe = rewind.nearestAtOrBefore(frame);
+        if (keyframe is null)
+        {
+            throw new InvalidOperationException(
+                "frame " + frame + " is older than the retained rewind window (oldest keyframe "
+                + rewind.oldestFrame + ")");
+        }
+
+        machine.loadState(keyframe.state);
+        currentFrame = keyframe.frameIndex;
+
+        while (currentFrame < frame)
+        {
+            stepForwardOnce();
+        }
+    }
+
+    public bool stepBack()
+    {
+        if (currentFrame == 0)
+        {
+            return false;
+        }
+        rewindTo(currentFrame - 1);
+        return true;
+    }
+
+    public byte[] currentState() => machine.saveState();
+
+    public ReplayOutcome outcome() => ReplayOutcome.capture(machine, currentFrame);
+
+    void stepForwardOnce()
+    {
+        KeyMask injected = movie.keysAt(currentFrame);
+        injected.applyTo(machine.keypad);
+        machine.runFrame(instructionsPerFrame);
+        currentFrame++;
+    }
+
+    void maybeRecordKeyframe()
+    {
+        if (currentFrame >= nextKeyframeFrame)
+        {
+            rewind.store(currentFrame, machine.saveState());
+            nextKeyframeFrame = currentFrame + keyframeInterval;
+        }
+    }
+}