diff --git a/src/models/event-timeline-set.ts b/src/models/event-timeline-set.ts
index 5cb04997e8b..9df897b182c 100644
--- a/src/models/event-timeline-set.ts
+++ b/src/models/event-timeline-set.ts
@@ -756,6 +756,99 @@ export class EventTimelineSet extends TypedEventEmitter<EmittedEvents, EventTime
         this.emit(RoomEvent.Timeline, event, this.room, Boolean(toStartOfTimeline), false, data);
     }
 
+    /**
+     * Insert event to the given timeline, and emit Room.timeline. Assumes
+     * we have already checked we don't know about this event.
+     *
+     * TEMPORARY: until we have recursive relations, we need this function
+     * to exist to allow us to insert events in timeline order, which is our
+     * best guess for Sync Order.
+     * This is a copy of addEventToTimeline above, modified to insert the event
+     * after the event it relates to, and before any event with a later
+     * timestamp. This is our best guess at Sync Order.
+     *
+     * Will fire "Room.timeline" for each event added.
+     *
+     * @param options - addEventToTimeline options
+     *
+     * @remarks
+     * Fires {@link RoomEvent.Timeline}
+     */
+    public insertEventIntoTimeline(event: MatrixEvent, timeline: EventTimeline, roomState: RoomState): void {
+        if (timeline.getTimelineSet() !== this) {
+            throw new Error(`EventTimelineSet.addEventToTimeline: Timeline=${timeline.toString()} does not belong " +
+                "in timelineSet(threadId=${this.thread?.id})`);
+        }
+
+        // Make sure events don't get mixed in timelines they shouldn't be in (e.g. a
+        // threaded message should not be in the main timeline).
+        //
+        // We can only run this check for timelines with a `room` because `canContain`
+        // requires it
+        if (this.room && !this.canContain(event)) {
+            let eventDebugString = `event=${event.getId()}`;
+            if (event.threadRootId) {
+                eventDebugString += `(belongs to thread=${event.threadRootId})`;
+            }
+            logger.warn(
+                `EventTimelineSet.addEventToTimeline: Ignoring ${eventDebugString} that does not belong ` +
+                    `in timeline=${timeline.toString()} timelineSet(threadId=${this.thread?.id})`,
+            );
+            return;
+        }
+
+        // Find the event that this event is related to - the "parent"
+        const parentEventId = event.relationEventId;
+        if (!parentEventId) {
+            // Not related to anything - we just append
+            this.addEventToTimeline(event, timeline, {
+                toStartOfTimeline: false,
+                fromCache: false,
+                timelineWasEmpty: false,
+                roomState,
+            });
+            return;
+        }
+
+        const parentEvent = this.findEventById(parentEventId);
+        if (!parentEvent) {
+            // Not related to anything we know about - just append
+            this.addEventToTimeline(event, timeline, {
+                toStartOfTimeline: false,
+                fromCache: false,
+                timelineWasEmpty: false,
+                roomState,
+            });
+            return;
+        }
+
+        const timelineEvents = timeline.getEvents();
+        const parentIndex = timelineEvents.indexOf(parentEvent);
+        let insertIndex = parentIndex;
+        for (; insertIndex < timelineEvents.length; insertIndex++) {
+            const nextEvent = timelineEvents[insertIndex];
+            if (nextEvent.getTs() > event.getTs()) {
+                // We found an event later than ours, so insert before that.
+                break;
+            }
+        }
+        // If we got to the end of the loop, insertIndex points at the end of
+        // the list.
+
+        const eventId = event.getId()!;
+        timeline.insertEvent(event, insertIndex, roomState);
+        this._eventIdToTimeline.set(eventId, timeline);
+
+        this.relations.aggregateParentEvent(event);
+        this.relations.aggregateChildEvent(event, this);
+
+        const data: IRoomTimelineData = {
+            timeline: timeline,
+            liveEvent: timeline == this.liveTimeline,
+        };
+        this.emit(RoomEvent.Timeline, event, this.room, false, false, data);
+    }
+
     /**
      * Replaces event with ID oldEventId with one with newEventId, if oldEventId is
      * recognised.  Otherwise, add to the live timeline.  Used to handle remote echos.
diff --git a/src/models/event-timeline.ts b/src/models/event-timeline.ts
index d1ba3210365..24f63a99965 100644
--- a/src/models/event-timeline.ts
+++ b/src/models/event-timeline.ts
@@ -427,6 +427,43 @@ export class EventTimeline {
         }
     }
 
+    /**
+     * Insert a new event into the timeline, and update the state.
+     *
+     * TEMPORARY: until we have recursive relations, we need this function
+     * to exist to allow us to insert events in timeline order, which is our
+     * best guess for Sync Order.
+     * This is a copy of addEvent above, modified to allow inserting an event at
+     * a specific index.
+     */
+    public insertEvent(event: MatrixEvent, insertIndex: number, roomState: RoomState): void {
+        const timelineSet = this.getTimelineSet();
+
+        if (timelineSet.room) {
+            EventTimeline.setEventMetadata(event, roomState, false);
+
+            // modify state but only on unfiltered timelineSets
+            if (event.isState() && timelineSet.room.getUnfilteredTimelineSet() === timelineSet) {
+                roomState.setStateEvents([event], {});
+                // it is possible that the act of setting the state event means we
+                // can set more metadata (specifically sender/target props), so try
+                // it again if the prop wasn't previously set. It may also mean that
+                // the sender/target is updated (if the event set was a room member event)
+                // so we want to use the *updated* member (new avatar/name) instead.
+                //
+                // However, we do NOT want to do this on member events if we're going
+                // back in time, else we'll set the .sender value for BEFORE the given
+                // member event, whereas we want to set the .sender value for the ACTUAL
+                // member event itself.
+                if (!event.sender || event.getType() === EventType.RoomMember) {
+                    EventTimeline.setEventMetadata(event, roomState!, false);
+                }
+            }
+        }
+
+        this.events.splice(insertIndex, 0, event); // insert element
+    }
+
     /**
      * Remove an event from the timeline
      *
diff --git a/src/models/thread.ts b/src/models/thread.ts
index c31499b2b34..fb7d84d98c1 100644
--- a/src/models/thread.ts
+++ b/src/models/thread.ts
@@ -236,6 +236,31 @@ export class Thread extends ReadReceipt<EmittedEvents, EventHandlerMap> {
         }
     }
 
+    /**
+     * TEMPORARY. Only call this when MSC3981 is not available, and we have some
+     * late-arriving events to insert, because we recursively found them as part
+     * of populating a thread. When we have MSC3981 we won't need it, because
+     * they will all be supplied by the homeserver in one request, and they will
+     * already be in the right order in that response.
+     * This is a copy of addEventToTimeline above, modified to call
+     * insertEventIntoTimeline so this event is inserted into our best guess of
+     * the right place based on timestamp. (We should be using Sync Order but we
+     * don't have it.)
+     */
+    public insertEventIntoTimeline(event: MatrixEvent): void {
+        const eventId = event.getId();
+        if (!eventId) {
+            return;
+        }
+        if (this.findEventById(eventId)) {
+            return;
+        }
+        this.timelineSet.insertEventIntoTimeline(event, this.liveTimeline, this.roomState);
+
+        // As far as we know, timeline should always be the same as events
+        this.timeline = this.events;
+    }
+
     public addEvents(events: MatrixEvent[], toStartOfTimeline: boolean): void {
         events.forEach((ev) => this.addEvent(ev, toStartOfTimeline, false));
         this.updateThreadMetadata();
@@ -281,7 +306,8 @@ export class Thread extends ReadReceipt<EmittedEvents, EventHandlerMap> {
                  */
                 this.replayEvents?.push(event);
             } else {
-                this.addEventToTimeline(event, toStartOfTimeline);
+                // TODO: check with Germain is this right?
+                this.insertEventIntoTimeline(event);
             }
             // Apply annotations and replace relations to the relations of the timeline only
             this.timelineSet.relations?.aggregateParentEvent(event);
@@ -473,6 +499,8 @@ export class Thread extends ReadReceipt<EmittedEvents, EventHandlerMap> {
                             .then((relations) => {
                                 if (relations.events.length) {
                                     event.makeReplaced(relations.events[0]);
+                                    // TODO: check with Germain: is this right?
+                                    this.insertEventIntoTimeline(event);
                                 }
                             })
                             .catch((e) => {