Arriving on the planet Dido in the 25th-century, the time travellers come upon a crashed spaceship from Earth. Its two occupants - a paralysed man, Bennett (Ray Barrett), and a young girl, Vicki (Maureen O'Brien) - are living in fear of the imposing figure of Koquillion, a native of the planet, whose people have apparently killed the other members of the human expedition. However, it does not take the Doctor long to deduce that Koquillion is in fact Bennett in disguise; it was actually he who killed the others in order to conceal an earlier murder he had committed on the ship. Confronted by two of the humanoid Didonians (John Stuart, Colin Hughes) - whom he thought he had completely wiped out - Bennett falls from a high rock ledge to his death.

On discovering that Vicki's father was amongst the murdered crewmen, and that consequently she is now an orphan, the Doctor offers her a place aboard the TARDIS, which she gratefully accepts.

_{Synopsis from Doctor Who: The First Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}