Having been displaced across a 'time track' the TARDIS with the Doctor et al arrives on the planet Xeros. There the travellers find their future selves displayed in a museum! The museum is actually a monument to the conquests of belligerent invaders (Moroks) who now rule Xeros. When time shifts back to normal, the Doctor realises that he and his companions have to do whatever they can to try to avert this future. They help the native people of Xeros to revolt against the Moroks. The revolution succeeds, the future is changed, and the travellers depart in the TARDIS.
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