The Space Museum

From http://www.bbc.co.uk/doctorwho/classic/episodeguide:
The TARDIS jumps a time track and the travellers arrive on the planet Xeros. There they discover their own future selves displayed as exhibits in a museum established as a monument to the galactic conquests of the warlike Morok invaders who now rule the planet. When time shifts back to normal, they realise that they must do everything they can to try to avert this potential future.

Vicki helps the native Xerons to obtain arms and thereby to revolt against the Moroks. The revolution succeeds and the travellers go on their way, confident that the future has been changed.

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