The Doctor is put on trial by the Time Lords again. The first of two events from his resent past is presented as evidence of his interference in the affairs of other worlds. In particular, his actions on planet Ravalox.

The Doctor finds that the subterranean UK Habitat is run by a cloistered robot named Drathro, whom the inhabitants call "The Immortal." Their black light energy converters are destabilizing, unaware that it's the result of two space pirates mucking about on the surface with designs on looting their dwelling.

The Doctor escapes to rejoin Peri on the surface but shortly becomes the prisoner of a surface-dwelling tribe called The Free. Ravalox isn't Ravalox after all, but before the Doctor can learn much more an irresistible agent from UK Habitat comes to fetch him back.

All those on Ravolox are in exponentially greater peril than anyone can comprehend. If anything in the entire universe is to survive, it may come down to the Doctor arguing against machine logic over the value of life.