To escape the destruction of a medical vessel the Sixth Doctor sends his red-headed companion Melanie Bush out in an escape pod, promising to seek her out once he rescues his TARDIS from the exploding ship. However, on finding his ship he is trapped in a Dalek time scoop. Mel, however, lands on the planet Lethe where her skills as a computer technician are keenly sought, and she joins a team led by the wheelchair-using Dr. Vaso and works to create the Juggernauts: "the ultimate service robots". She becomes quite attached to the team (catching the eye of the juvenile but intelligent Geoff) and particularly the elderly Vaso, with whom she strikes up a rapport.
The Doctor however, discovers himself employed by the Daleks to spy on the situation on Lethe. He agrees; for they threaten Mel's life. What is the secret of the Juggernauts? Why are the Daleks interested in Lethe? What are the strange things residing in the darkness of the lower echelons of the colony?