diff --git a/game/main.tscn b/game/main.tscn
new file mode 100644
index 0000000..b54e251
--- /dev/null
+++ b/game/main.tscn
@@ -0,0 +1,6 @@
+[gd_scene format=3]
+
+[ext_resource type="Script" path="res://scripts/main.gd" id="1_main"]
+
+[node name="Main" type="Node2D"]
+script = ExtResource("1_main")
diff --git a/game/project.godot b/game/project.godot
index ddf4ef3..4a8f097 100644
--- a/game/project.godot
+++ b/game/project.godot
@@ -7,6 +7,7 @@ config_version=5
 [application]
 
 config/name="Grovewright"
+run/main_scene="res://main.tscn"
 config/features=PackedStringArray("4.7", "GL Compatibility")
 
 [display]
diff --git a/game/scripts/hud.gd b/game/scripts/hud.gd
new file mode 100644
index 0000000..e81b625
--- /dev/null
+++ b/game/scripts/hud.gd
@@ -0,0 +1,58 @@
+class_name Hud
+extends CanvasLayer
+## Text over the game: the stone count, short messages and a help line.
+
+const MESSAGE_SECONDS := 2.5
+
+var _stone := Label.new()
+var _message := Label.new()
+var _help := Label.new()
+var _message_left := 0.0
+
+
+func _ready() -> void:
+	_stone.position = Vector2(16, 12)
+	_stone.add_theme_font_size_override("font_size", 22)
+	_message.position = Vector2(16, 48)
+	_message.add_theme_font_size_override("font_size", 18)
+	_help.text = tr("HELP")
+	_help.add_theme_font_size_override("font_size", 14)
+	for label: Label in [_stone, _message, _help]:
+		label.add_theme_color_override("font_outline_color", Color.BLACK)
+		label.add_theme_constant_override("outline_size", 4)
+		add_child(label)
+	_help.set_anchors_and_offsets_preset(Control.PRESET_BOTTOM_LEFT, Control.PRESET_MODE_MINSIZE, 16)
+
+
+func _process(delta: float) -> void:
+	if _message_left <= 0.0:
+		return
+	_message_left -= delta
+	if _message_left <= 0.0:
+		_message.text = ""
+
+
+func show_stone(count: int) -> void:
+	_stone.text = tr("STONE_COUNT") % count
+
+
+## Shows the text for `key` for a few seconds.
+func flash(key: String) -> void:
+	_message.text = tr(key)
+	_message_left = MESSAGE_SECONDS
+
+
+## Shows or hides the pause notice. It stays until the game continues.
+func show_paused(paused: bool) -> void:
+	_message.text = tr("PAUSED") if paused else ""
+	_message_left = 0.0
+
+
+## The text currently shown as a message. Tests read it.
+func message() -> String:
+	return _message.text
+
+
+## The text currently shown as the stone count. Tests read it.
+func stone_text() -> String:
+	return _stone.text
diff --git a/game/scripts/hud.gd.uid b/game/scripts/hud.gd.uid
new file mode 100644
index 0000000..5cc06a0
--- /dev/null
+++ b/game/scripts/hud.gd.uid
@@ -0,0 +1 @@
+uid://cosq3mrn0ve5v
diff --git a/game/scripts/main.gd b/game/scripts/main.gd
new file mode 100644
index 0000000..8ef9828
--- /dev/null
+++ b/game/scripts/main.gd
@@ -0,0 +1,162 @@
+extends Node2D
+## Composition root of a solo session: connects the simulation core with the
+## map view, the figure, the HUD and the files on disk.
+
+## Seed of a new world until the slice ships its chosen seed.
+const NEW_WORLD_SEED := 2026
+const PLAYER_NAME := "Spieler"
+## How far the pickaxe reaches, in tiles from the figure to the tile centre.
+const REACH_TILES := 2.5
+## Ticks between two automatic saves: two minutes.
+const AUTOSAVE_TICKS := 2400
+const CAMERA_ZOOM := Vector2(3.0, 3.0)
+
+## Folder for the player key, the save and the recordings. Tests use their
+## own folder.
+@export var storage_dir := "user://"
+
+var session: GwSession
+var store: SaveStore
+var clock: TickClock
+var map_view: MapView
+var marker: TileMarker
+var avatar: Avatar
+var hud: Hud
+
+var _player_key: PackedByteArray
+var _next_dig_tick := 0
+
+
+func _ready() -> void:
+	I18n.install()
+	InputSetup.install()
+	store = SaveStore.new(storage_dir)
+	_player_key = store.player_key()
+	clock = TickClock.new(GwSession.TICKS_PER_SECOND)
+	map_view = MapView.new()
+	add_child(map_view)
+	marker = TileMarker.new()
+	add_child(marker)
+	avatar = Avatar.new()
+	add_child(avatar)
+	var camera := Camera2D.new()
+	camera.zoom = CAMERA_ZOOM
+	avatar.add_child(camera)
+	hud = Hud.new()
+	add_child(hud)
+	if not _start_from_save():
+		_start_new_world()
+
+
+func _process(delta: float) -> void:
+	if session == null or session.is_broken():
+		return
+	_update_marker()
+	if Input.is_action_pressed("dig") and not clock.paused:
+		_try_dig()
+	for i in clock.advance(delta):
+		_run_tick()
+
+
+func _unhandled_input(event: InputEvent) -> void:
+	if event.is_action_pressed("save_game"):
+		hud.flash("SAVED" if save_game() else "SAVE_FAILED")
+	elif event.is_action_pressed("load_game"):
+		_write_recording()
+		hud.flash("LOADED" if _start_from_save() else "LOAD_FAILED")
+	elif event.is_action_pressed("toggle_pause"):
+		clock.paused = not clock.paused
+		avatar.set_process(not clock.paused)
+		hud.show_paused(clock.paused)
+
+
+func _notification(what: int) -> void:
+	if what == NOTIFICATION_WM_CLOSE_REQUEST and session != null:
+		save_game()
+		_write_recording()
+
+
+func _start_new_world() -> void:
+	var fresh := GwSession.new()
+	fresh.start_new(NEW_WORLD_SEED)
+	_enter(fresh)
+
+
+## Continues the saved world. Returns false if there is no usable save. An
+## unusable save is set aside, so that the next save does not overwrite it.
+func _start_from_save() -> bool:
+	if not store.has_save():
+		return false
+	var loaded := GwSession.new()
+	if not loaded.start_loaded(store.read_save()):
+		var aside := store.set_aside_save()
+		push_warning("save not usable (%s), kept as %s" % [loaded.last_error(), aside])
+		hud.flash("LOAD_FAILED")
+		return false
+	_enter(loaded)
+	return true
+
+
+## Makes `next` the running session and lets the local player join it.
+func _enter(next: GwSession) -> void:
+	session = next
+	session.join(_player_key, PLAYER_NAME)
+	_run_tick()
+	_next_dig_tick = 0
+	map_view.session = session
+	map_view.queue_redraw()
+	avatar.is_walkable = session.is_walkable
+	avatar.position = GridMotion.tile_center(session.spawn_point())
+	hud.show_stone(session.item_count(GwSession.ITEM_STONE))
+
+
+func _run_tick() -> void:
+	_apply(session.step())
+	if session.tick() % AUTOSAVE_TICKS == 0:
+		save_game()
+
+
+func _apply(events: Array[Dictionary]) -> void:
+	var local := session.local_player()
+	for event in events:
+		match event["type"]:
+			"tile_hit", "tile_changed":
+				map_view.queue_redraw()
+			"item_gained":
+				if event["player"] == local:
+					hud.show_stone(session.item_count(GwSession.ITEM_STONE))
+			"inventory_full":
+				if event["player"] == local:
+					hud.flash("INVENTORY_FULL")
+	if session.is_broken():
+		hud.flash("SIMULATION_STOPPED")
+		push_error(session.last_error())
+
+
+func _update_marker() -> void:
+	var tile := GridMotion.tile_of(get_global_mouse_position())
+	var distance := avatar.position.distance_to(GridMotion.tile_center(tile))
+	marker.show_tile(tile, distance <= REACH_TILES * GridMotion.TILE_SIZE)
+
+
+func _try_dig() -> void:
+	if marker.in_reach:
+		dig_at(marker.tile)
+
+
+## Queues a pickaxe hit on `tile`, at most once per cooldown. The core checks
+## everything else.
+func dig_at(tile: Vector2i) -> void:
+	if session.tick() < _next_dig_tick:
+		return
+	session.dig(tile.x, tile.y)
+	_next_dig_tick = session.tick() + GwSession.DIG_COOLDOWN_TICKS
+
+
+## Writes the running world as the save. Returns false if nothing was written.
+func save_game() -> bool:
+	return store.write_save(session.save())
+
+
+func _write_recording() -> void:
+	store.write_recording(session.recording_bytes())
diff --git a/game/scripts/main.gd.uid b/game/scripts/main.gd.uid
new file mode 100644
index 0000000..c0369b6
--- /dev/null
+++ b/game/scripts/main.gd.uid
@@ -0,0 +1 @@
+uid://by56f6vi542a1
diff --git a/game/scripts/map_view.gd b/game/scripts/map_view.gd
new file mode 100644
index 0000000..f9913d0
--- /dev/null
+++ b/game/scripts/map_view.gd
@@ -0,0 +1,48 @@
+class_name MapView
+extends Node2D
+## Draws the terrain of the shared world. The colours are placeholders until
+## a style is chosen from the samples of plan M1d.
+
+const CRACK_COLOR := Color(0.1, 0.08, 0.06, 0.8)
+
+var session: GwSession
+
+
+func _draw() -> void:
+	if session == null or not session.is_running():
+		return
+	var width := session.map_width()
+	var height := session.map_height()
+	var terrain := session.terrain_codes()
+	var damage := session.damage_codes()
+	var size := GridMotion.TILE_SIZE
+	for y in height:
+		for x in width:
+			var index := y * width + x
+			var rect := Rect2(x * size, y * size, size, size)
+			draw_rect(rect, color_of(terrain[index]))
+			if damage[index] > 0:
+				_draw_cracks(rect, damage[index])
+
+
+## The placeholder colour of a terrain code.
+static func color_of(terrain: int) -> Color:
+	if terrain == GwSession.TERRAIN_GROUND:
+		return Color(0.42, 0.36, 0.28)
+	if terrain == GwSession.TERRAIN_SOFT_ROCK:
+		return Color(0.56, 0.55, 0.52)
+	if terrain == GwSession.TERRAIN_HARD_ROCK:
+		return Color(0.33, 0.33, 0.37)
+	if terrain == GwSession.TERRAIN_BEDROCK:
+		return Color(0.12, 0.12, 0.14)
+	return Color.MAGENTA
+
+
+## One crack line for every hit the tile has taken.
+func _draw_cracks(rect: Rect2, hits: int) -> void:
+	var inset := rect.grow(-3.0)
+	for line in hits:
+		var share := (line + 1.0) / (hits + 1.0)
+		var from := Vector2(inset.position.x, inset.position.y + inset.size.y * share)
+		var to := Vector2(inset.end.x, inset.position.y + inset.size.y * (1.0 - share))
+		draw_line(from, to, CRACK_COLOR, 1.0)
diff --git a/game/scripts/map_view.gd.uid b/game/scripts/map_view.gd.uid
new file mode 100644
index 0000000..d2926de
--- /dev/null
+++ b/game/scripts/map_view.gd.uid
@@ -0,0 +1 @@
+uid://dasl7vpyifxob
diff --git a/game/scripts/tile_marker.gd b/game/scripts/tile_marker.gd
new file mode 100644
index 0000000..123fc9b
--- /dev/null
+++ b/game/scripts/tile_marker.gd
@@ -0,0 +1,25 @@
+class_name TileMarker
+extends Node2D
+## Outlines the tile under the mouse: green when the figure can reach it,
+## red when it is too far away.
+
+const REACHABLE_COLOR := Color(0.4, 1.0, 0.4)
+const UNREACHABLE_COLOR := Color(1.0, 0.4, 0.4)
+
+var tile := Vector2i(-1, -1)
+var in_reach := false
+
+
+## Moves the outline to `new_tile`. Redraws only when something changed.
+func show_tile(new_tile: Vector2i, reachable: bool) -> void:
+	if new_tile == tile and reachable == in_reach:
+		return
+	tile = new_tile
+	in_reach = reachable
+	queue_redraw()
+
+
+func _draw() -> void:
+	var size := GridMotion.TILE_SIZE
+	var color := REACHABLE_COLOR if in_reach else UNREACHABLE_COLOR
+	draw_rect(Rect2(Vector2(tile) * size, Vector2(size, size)), color, false, 1.0)
diff --git a/game/scripts/tile_marker.gd.uid b/game/scripts/tile_marker.gd.uid
new file mode 100644
index 0000000..cd3db8e
--- /dev/null
+++ b/game/scripts/tile_marker.gd.uid
@@ -0,0 +1 @@
+uid://cjnucbgwr65ud
diff --git a/game/tests/test_main.gd b/game/tests/test_main.gd
new file mode 100644
index 0000000..103d87f
--- /dev/null
+++ b/game/tests/test_main.gd
@@ -0,0 +1,60 @@
+extends TestCase
+## Starts the real main scene with its own storage folder and runs a few
+## frames.
+
+
+func _start_main(directory: String) -> Node:
+	var main: Node = load("res://main.tscn").instantiate()
+	main.storage_dir = directory
+	tree.root.add_child(main)
+	return main
+
+
+func _stop_main(main: Node) -> void:
+	tree.root.remove_child(main)
+	main.free()
+
+
+func test_a_new_game_starts_and_the_player_joins() -> void:
+	var directory := TestFiles.temporary_dir("main")
+	var main := _start_main(directory)
+	for frame in 5:
+		await tree.process_frame
+	var session: GwSession = main.session
+	check(session != null and session.is_running(), "a session runs")
+	check(not session.is_broken(), "the simulation is healthy")
+	check_eq(session.local_player(), 0, "the local player has joined")
+	check_eq(main.avatar.position, GridMotion.tile_center(session.spawn_point()), "the figure stands on the spawn")
+	check_eq(main.hud.stone_text(), tr("STONE_COUNT") % 0, "the HUD shows the stones")
+	_stop_main(main)
+	TestFiles.remove_dir(directory)
+
+
+func test_a_saved_game_is_continued_on_the_next_start() -> void:
+	var directory := TestFiles.temporary_dir("continue")
+	var first := _start_main(directory)
+	await tree.process_frame
+	var tick_at_save: int = first.session.tick()
+	check(first.save_game(), "the game was saved")
+	_stop_main(first)
+
+	var second := _start_main(directory)
+	await tree.process_frame
+	check(second.session.tick() > tick_at_save, "the saved world continues")
+	check_eq(second.session.local_player(), 0, "the same player rejoins")
+	_stop_main(second)
+	TestFiles.remove_dir(directory)
+
+
+func test_an_unusable_save_is_set_aside_and_a_new_world_starts() -> void:
+	var directory := TestFiles.temporary_dir("broken")
+	var store := SaveStore.new(directory)
+	store.write_save(PackedByteArray([1, 2, 3]))
+	var main := _start_main(directory)
+	await tree.process_frame
+	check(main.session.is_running(), "a new world runs")
+	check(main.session.tick() <= 5, "it is new, tick %d" % main.session.tick())
+	check(FileAccess.file_exists(directory.path_join(SaveStore.SAVE_FILE + ".broken")), "the unusable save is kept aside")
+	check_eq(main.hud.message(), tr("LOAD_FAILED"), "the player is told")
+	_stop_main(main)
+	TestFiles.remove_dir(directory)
diff --git a/game/tests/test_main.gd.uid b/game/tests/test_main.gd.uid
new file mode 100644
index 0000000..04807e2
--- /dev/null
+++ b/game/tests/test_main.gd.uid
@@ -0,0 +1 @@
+uid://d0qbyofproc3j
diff --git a/game/tools/screenshot.gd b/game/tools/screenshot.gd
new file mode 100644
index 0000000..b417a71
--- /dev/null
+++ b/game/tools/screenshot.gd
@@ -0,0 +1,50 @@
+extends SceneTree
+## Starts the game in a temporary storage folder, walks the figure toward the
+## rock beside the clearing, hits it once and saves a screenshot. Lets a
+## change to the look be checked without playing by hand. The player's real
+## key and save stay untouched.
+##
+## From the repository root (opens a window for a moment):
+##   godot --path game -s res://tools/screenshot.gd -- <path of the PNG>
+
+const STORAGE := "user://screenshot-run"
+const FRAMES_AFTER_HIT := 30
+
+
+func _initialize() -> void:
+	_run.call_deferred()
+
+
+func _run() -> void:
+	var arguments := OS.get_cmdline_user_args()
+	var output := arguments[0] if arguments.size() > 0 else "user://screenshot.png"
+	DirAccess.make_dir_recursive_absolute(STORAGE)
+	var main: Node = load("res://main.tscn").instantiate()
+	main.storage_dir = STORAGE
+	root.add_child(main)
+	await process_frame
+
+	# Four tiles to the right, toward the rock outcrop beside the clearing.
+	for frame in 48:
+		main.avatar.walk(Vector2.RIGHT, 1.0 / 60.0)
+		await process_frame
+	var session: GwSession = main.session
+	var rock := session.spawn_point() + Vector2i(7, 0)
+	main.dig_at(rock)
+	for frame in FRAMES_AFTER_HIT:
+		await process_frame
+
+	var image := root.get_texture().get_image()
+	var error := image.save_png(output)
+	print("screenshot %s: %s" % ["saved" if error == OK else "failed", ProjectSettings.globalize_path(output)])
+	root.remove_child(main)
+	main.free()
+	for file in DirAccess.get_files_at(STORAGE):
+		DirAccess.remove_absolute(STORAGE.path_join(file))
+	for directory in DirAccess.get_directories_at(STORAGE):
+		var path := STORAGE.path_join(directory)
+		for file in DirAccess.get_files_at(path):
+			DirAccess.remove_absolute(path.path_join(file))
+		DirAccess.remove_absolute(path)
+	DirAccess.remove_absolute(STORAGE)
+	quit(0 if error == OK else 1)
diff --git a/game/tools/screenshot.gd.uid b/game/tools/screenshot.gd.uid
new file mode 100644
index 0000000..f2834e2
--- /dev/null
+++ b/game/tools/screenshot.gd.uid
@@ -0,0 +1 @@
+uid://cltfxvihioc4v