The TARDIS has finally brought the Doctor and Sam back to Earth - and straight into danger.
It is 1963. Six very different people have been gathered together for study by parapsychologist Charles Roley in his stately home outside London. All of them claim to have been possessed by the devil, and all have shared similar delusions - they describe the same bizarre 'death cave' riddled with demons.
Roley's experiments are having a gradual yet terrifying effect on his subjects, and the Doctor and Sam discover the connections between those tainted with the madness are more disturbing than anyone could guess.
For the Doctor, too, has seen the cave they describe - on a dead world, billions of years ago...