diff --git a/backend/app/services/narrative/world_state.py b/backend/app/services/narrative/world_state.py
new file mode 100644
index 00000000..510dc279
--- /dev/null
+++ b/backend/app/services/narrative/world_state.py
@@ -0,0 +1,59 @@
+"""World state CRUD: rules, locations, event log.
+
+Stored in narrative/world_state.json. Missing file is treated as an empty
+world so existing simulations (from pre-God-Mode versions) continue to work
+without migration.
+"""
+import os
+import json
+
+
+class WorldStateStore:
+    """Manages narrative/world_state.json for a single simulation."""
+
+    def __init__(self, sim_dir: str):
+        self.sim_dir = sim_dir
+        self.narrative_dir = os.path.join(sim_dir, "narrative")
+        self.path = os.path.join(self.narrative_dir, "world_state.json")
+
+    def _ensure_dir(self) -> None:
+        os.makedirs(self.narrative_dir, exist_ok=True)
+
+    def load(self) -> dict:
+        if not os.path.exists(self.path):
+            return {"rules": [], "locations": {}, "event_log": []}
+        with open(self.path, "r", encoding="utf-8") as f:
+            world = json.load(f)
+        # Fill in missing keys for forward compatibility
+        world.setdefault("rules", [])
+        world.setdefault("locations", {})
+        world.setdefault("event_log", [])
+        return world
+
+    def save(self, world: dict) -> None:
+        self._ensure_dir()
+        with open(self.path, "w", encoding="utf-8") as f:
+            json.dump(world, f, ensure_ascii=False, indent=2)
+
+    def set_rules(self, rules: list[str]) -> None:
+        world = self.load()
+        world["rules"] = list(rules)
+        self.save(world)
+
+    def upsert_location(self, location: dict) -> dict:
+        """Insert or update a location by id. Returns the stored entry."""
+        if "id" not in location:
+            raise ValueError("location requires 'id'")
+        world = self.load()
+        world["locations"][location["id"]] = location
+        self.save(world)
+        return location
+
+    def append_event(self, event: dict) -> dict:
+        """Append an event to event_log, assigning evt_N id automatically."""
+        world = self.load()
+        event = dict(event)
+        event["id"] = f"evt_{len(world['event_log']) + 1}"
+        world["event_log"].append(event)
+        self.save(world)
+        return event
diff --git a/backend/tests/test_world_state.py b/backend/tests/test_world_state.py
new file mode 100644
index 00000000..d5ae49d7
--- /dev/null
+++ b/backend/tests/test_world_state.py
@@ -0,0 +1,45 @@
+import os
+import tempfile
+import pytest
+from app.services.narrative.world_state import WorldStateStore
+
+
+@pytest.fixture
+def temp_sim_dir():
+    with tempfile.TemporaryDirectory() as d:
+        sim_dir = os.path.join(d, "sim_test")
+        os.makedirs(sim_dir)
+        yield sim_dir
+
+
+def test_load_returns_empty_world_when_missing(temp_sim_dir):
+    store = WorldStateStore(temp_sim_dir)
+    world = store.load()
+    assert world == {"rules": [], "locations": {}, "event_log": []}
+
+
+def test_set_rules_replaces_previous(temp_sim_dir):
+    store = WorldStateStore(temp_sim_dir)
+    store.set_rules(["rule 1", "rule 2"])
+    assert store.load()["rules"] == ["rule 1", "rule 2"]
+    store.set_rules(["only rule"])
+    assert store.load()["rules"] == ["only rule"]
+
+
+def test_upsert_location_adds_and_updates(temp_sim_dir):
+    store = WorldStateStore(temp_sim_dir)
+    store.upsert_location({"id": "tower", "name": "The Tower", "description": "tall"})
+    assert store.load()["locations"]["tower"]["name"] == "The Tower"
+
+    store.upsert_location({"id": "tower", "name": "The Iron Tower", "description": "dark"})
+    assert store.load()["locations"]["tower"]["name"] == "The Iron Tower"
+
+
+def test_append_event_auto_ids_sequentially(temp_sim_dir):
+    store = WorldStateStore(temp_sim_dir)
+    e1 = store.append_event({"type": "custom", "description": "one", "round": 1})
+    e2 = store.append_event({"type": "custom", "description": "two", "round": 2})
+
+    assert e1["id"] == "evt_1"
+    assert e2["id"] == "evt_2"
+    assert store.load()["event_log"][-1]["description"] == "two"