When United Nations vessels carrying radioactive isotopes are attacked by war machines from different eras, the Doctor and Jo are called in to investigate. The answer to this mystery lies far away, in a race under the sway of the Master.
According to one account, the Master had the appearance of Roger Delgado while on Gallifrey. According to another, he had Anthony Ainley's likeness. According to another one, the one with James Dreyfus's appearance was the incarnation who ran away from Gallifrey.
While fighting to extend his life at the end of his regeneration cycle, many bodies were possessed by the Beevers incarnation, but all kept somehow reverting to his real being until he finally regenerated into the MacQueen one. Hence, these sections cannot be strictly chronological
