Having left Susan on Earth, the Doctor, Ian and Barbara land the TARDIS on the planet Dido in the year 2493. There they discover two humans, Bennett and Vicki, trapped in their spaceship, the only crew to survive death at the hands of the hostile Didonians.
But the Doctor is baffled: the Didonians are a peaceful nation. What has happened to change them? Why have they murdered the crew of the spaceship? Why, apart from the mysterious Koquillion, are they strangely absent?
The Doctor must find the answers - and quickly. For a rescue ship is on its way from Earth intent on revenge and time is running out for the planet.