There has been a series of mysterious vanishings on the desert planet Chronos in the 33rd century. Survey teams working for a university seem to have simply vanished amongst the pyramids on the planet. Alongside two other survey teams and an expert on cybernetics, the Doctor and Evelyn learn the deadly truth: that the planet Chronos is being used as a base for one of the Doctor's oldest and deadliest foes — the Cybermen. (thetvdb.com)