While trying to fix the TARDIS's chameleon circuit, the Sixth Doctor returns to Foreman's Yard on Totter's Lane in 1985, where he meets his old enemies the Cybermen. They have come from the future to change history by sending Halley's Comet crashing into Earth. Lytton, last seen working for the Daleks, is involved in the Cybermen's plot. Is Lytton working for the Cybermen, himself or someone else?