diff --git a/clientapi/producers/roomserver.go b/clientapi/producers/roomserver.go
index a804abfe..7eee83f5 100644
--- a/clientapi/producers/roomserver.go
+++ b/clientapi/producers/roomserver.go
@@ -54,7 +54,7 @@ func (c *RoomserverProducer) SendEvents(
 // SendEventWithState writes an event with KindNew to the roomserver input log
 // with the state at the event as KindOutlier before it.
 func (c *RoomserverProducer) SendEventWithState(
-	ctx context.Context, state gomatrixserverlib.RespState, event gomatrixserverlib.HeaderedEvent,
+	ctx context.Context, state *gomatrixserverlib.RespState, event gomatrixserverlib.HeaderedEvent,
 ) error {
 	outliers, err := state.Events()
 	if err != nil {
diff --git a/federationapi/routing/send.go b/federationapi/routing/send.go
index 88411b81..a6072c66 100644
--- a/federationapi/routing/send.go
+++ b/federationapi/routing/send.go
@@ -26,6 +26,7 @@ import (
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
+	"github.com/sirupsen/logrus"
 )
 
 // Send implements /_matrix/federation/v1/send/{txnID}
@@ -297,22 +298,27 @@ func (t *txnReq) processEventWithMissingState(e gomatrixserverlib.Event, roomVer
 	// However not all version of synapse support /state_ids so you may
 	// need to fallback to /state.
 	// TODO: Attempt to fill in the gap using /get_missing_events
-	// TODO: Attempt to fetch the state using /state_ids and /events
-	state, err := t.federation.LookupState(t.context, t.Origin, e.RoomID(), e.EventID(), roomVersion)
+
+	// Attempt to fetch the missing state using /state_ids and /events
+	var respState *gomatrixserverlib.RespState
+	var err error
+	respState, err = t.lookupMissingStateViaStateIDs(e, roomVersion)
 	if err != nil {
-		return err
-	}
-	// Check that the returned state is valid.
-	if err := state.Check(t.context, t.keys); err != nil {
-		return err
+		// Fallback to /state
+		util.GetLogger(t.context).WithError(err).Warn("processEventWithMissingState failed to /state_ids, falling back to /state")
+		respState, err = t.lookupMissingStateViaState(e, roomVersion)
+		if err != nil {
+			return err
+		}
 	}
+
 	// Check that the event is allowed by the state.
 retryAllowedState:
-	if err := checkAllowedByState(e, state.StateEvents); err != nil {
+	if err := checkAllowedByState(e, respState.StateEvents); err != nil {
 		switch missing := err.(type) {
 		case gomatrixserverlib.MissingAuthEventError:
 			// An auth event was missing so let's look up that event over federation
-			for _, s := range state.StateEvents {
+			for _, s := range respState.StateEvents {
 				if s.EventID() != missing.AuthEventID {
 					continue
 				}
@@ -329,5 +335,113 @@ retryAllowedState:
 	}
 
 	// pass the event along with the state to the roomserver
-	return t.producer.SendEventWithState(t.context, state, e.Headered(roomVersion))
+	return t.producer.SendEventWithState(t.context, respState, e.Headered(roomVersion))
+}
+
+func (t *txnReq) lookupMissingStateViaState(e gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion) (
+	respState *gomatrixserverlib.RespState, err error) {
+	state, err := t.federation.LookupState(t.context, t.Origin, e.RoomID(), e.EventID(), roomVersion)
+	if err != nil {
+		return nil, err
+	}
+	// Check that the returned state is valid.
+	if err := state.Check(t.context, t.keys); err != nil {
+		return nil, err
+	}
+	return &state, nil
+}
+
+func (t *txnReq) lookupMissingStateViaStateIDs(e gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion) (
+	*gomatrixserverlib.RespState, error) {
+
+	// fetch all the state events we do know about: the current state
+	queryReq := api.QueryLatestEventsAndStateRequest{
+		RoomID:       e.RoomID(),
+		StateToFetch: nil, // fetch all state
+	}
+	var queryRes api.QueryLatestEventsAndStateResponse
+	if err := t.rsAPI.QueryLatestEventsAndState(t.context, &queryReq, &queryRes); err != nil {
+		return nil, err
+	}
+	if !queryRes.RoomExists {
+		return nil, fmt.Errorf("room %s doesn't exist", e.RoomID())
+	}
+	// allow indexing of current state by event ID
+	haveEventMap := make(map[string]*gomatrixserverlib.HeaderedEvent, len(queryRes.StateEvents))
+	for i := range queryRes.StateEvents {
+		haveEventMap[queryRes.StateEvents[i].EventID()] = &queryRes.StateEvents[i]
+	}
+
+	// fetch the state event IDs at the time of the event
+	stateIDs, err := t.federation.LookupStateIDs(t.context, t.Origin, e.RoomID(), e.EventID())
+	if err != nil {
+		return nil, err
+	}
+	// work out which auth/state IDs are missing
+	wantIDs := append(stateIDs.StateEventIDs, stateIDs.AuthEventIDs...)
+	missing := make(map[string]bool)
+	for _, sid := range wantIDs {
+		if _, ok := haveEventMap[sid]; !ok {
+			missing[sid] = true
+		}
+	}
+	util.GetLogger(t.context).WithFields(logrus.Fields{
+		"missing":           len(missing),
+		"event_id":          e.EventID(),
+		"room_id":           e.RoomID(),
+		"already_have":      len(queryRes.StateEvents),
+		"total_state":       len(stateIDs.StateEventIDs),
+		"total_auth_events": len(stateIDs.AuthEventIDs),
+	}).Info("Fetching missing state at event")
+
+	for missingEventID := range missing {
+		txn, err := t.federation.GetEvent(t.context, t.Origin, missingEventID)
+		if err != nil {
+			util.GetLogger(t.context).WithError(err).WithField("event_id", missingEventID).Warn("failed to get missing /event for event ID")
+			return nil, err
+		}
+		for _, pdu := range txn.PDUs {
+			event, err := gomatrixserverlib.NewEventFromUntrustedJSON(pdu, roomVersion)
+			if err != nil {
+				util.GetLogger(t.context).WithError(err).Warnf("Transaction: Failed to parse event JSON of event %q", event.EventID())
+				return nil, unmarshalError{err}
+			}
+			if err := gomatrixserverlib.VerifyAllEventSignatures(t.context, []gomatrixserverlib.Event{event}, t.keys); err != nil {
+				util.GetLogger(t.context).WithError(err).Warnf("Transaction: Couldn't validate signature of event %q", event.EventID())
+				return nil, verifySigError{event.EventID(), err}
+			}
+			h := event.Headered(roomVersion)
+			haveEventMap[event.EventID()] = &h
+		}
+	}
+	return t.createRespStateFromStateIDs(stateIDs, haveEventMap)
+}
+
+func (t *txnReq) createRespStateFromStateIDs(stateIDs gomatrixserverlib.RespStateIDs, haveEventMap map[string]*gomatrixserverlib.HeaderedEvent) (
+	*gomatrixserverlib.RespState, error) {
+	// create a RespState response using the response to /state_ids as a guide
+	respState := gomatrixserverlib.RespState{
+		AuthEvents:  make([]gomatrixserverlib.Event, len(stateIDs.AuthEventIDs)),
+		StateEvents: make([]gomatrixserverlib.Event, len(stateIDs.StateEventIDs)),
+	}
+
+	for i := range stateIDs.StateEventIDs {
+		ev, ok := haveEventMap[stateIDs.StateEventIDs[i]]
+		if !ok {
+			return nil, fmt.Errorf("missing state event %s", stateIDs.StateEventIDs[i])
+		}
+		respState.StateEvents[i] = ev.Unwrap()
+	}
+	for i := range stateIDs.AuthEventIDs {
+		ev, ok := haveEventMap[stateIDs.AuthEventIDs[i]]
+		if !ok {
+			return nil, fmt.Errorf("missing auth event %s", stateIDs.AuthEventIDs[i])
+		}
+		respState.AuthEvents[i] = ev.Unwrap()
+	}
+	// Check that the returned state is valid.
+	if err := respState.Check(t.context, t.keys); err != nil {
+		return nil, err
+	}
+	return &respState, nil
 }
diff --git a/roomserver/api/query.go b/roomserver/api/query.go
index cb7cbb86..9afc51f4 100644
--- a/roomserver/api/query.go
+++ b/roomserver/api/query.go
@@ -29,7 +29,7 @@ type QueryLatestEventsAndStateRequest struct {
 	// The room ID to query the latest events for.
 	RoomID string `json:"room_id"`
 	// The state key tuples to fetch from the room current state.
-	// If this list is empty or nil then no state events are returned.
+	// If this list is empty or nil then *ALL* current state events are returned.
 	StateToFetch []gomatrixserverlib.StateKeyTuple `json:"state_to_fetch"`
 }