The Doctor's TARDIS is stolen, and tracking it down reveals the existence of an alien named Kalid controlling events in the past. While the Doctor challenges Kalid's motives for being there, Tegan and Nyssa discover the source of Kalid's power - an even greater threat for the Doctor.

From within their sarcophagus, the gestalt Xeraphin are communicating through Nyssa. But their psychic energy is so powerful it may destroy her body completely.

The Master's TARDIS is repaired and the Doctor must follow him back to 20th century Heathrow, once he has rescued all the passengers and crew from both Concordes.