'Just how technologically advanced are they?'
The Doctor frowned. 'Let me put it this way: they have a non-aggression pact with the Time-Lords.'

The Doctor has taken his companions to paradise, or at least the closest thing he can find. A sun enclosed by an artificial sphere where there is no want, poverty, or violence.

While Chris learns to surf, meets a girl and falls in love with a biplane, Roz suspects an alien plot and Bernice considers that a Dyson Sphere needs an archeologist like a fish needs a five-speed gear box.

Then the peace is shattered by murder. As the suspects proliferate, Bernice realises that even an artificial world has its buried secrets and Roz discovers what she's always suspected - that every paradise has its snake.