The TARDIS brings the fourth Doctor, Romana and K-9 to the Rock Of Judgement: a court, prison and place of execution built into a rocket-powered asteroid. There they become embroiled in an investigation by the system's finest lawman.

What connects the macabre gallery of artist Menlove Stokes with the slaugther of a survey team on a distant planet? Why is Margo, chief of security, behaving so strangely? And which old enemies of the Doctor are aboard the unmarked spaceship making its way towards the Rock?