The Warlords
The first episode of the subsequent story, "The Space Museum", begins with a brief clip of the finale of Episode 4, "The Warlords", where the time-travellers stand in period costume round the TARDIS console, literally frozen in time (it is confirmed as a clip rather than a restaged scene by an off-camera cough on the soundtracks to both "The Warlords" and "The Space Museum"). This is the only known surviving footage from the final episode.