The War Games

From http://www.bbc.co.uk/doctorwho/classic/episodeguide:

The TARDIS arrives on a planet where a race known only as the Aliens have gathered soldiers from a number of different wars in Earth's history, brainwashed them and put them to battle. Their aim is to form an invincible army from the survivors and use this to take over the galaxy.

The War Lord is assisted by a Security Chief and a War Chief, the latter of whom the Doctor quickly recognises as a member of his own race, the Time Lords. The War Chief has provided the Aliens with the time vessels, SIDRATs, that are essential to their scheme; but he secretly plans to double-cross them and seize power for himself.

When the War Lord learns of the War Chief's duplicity he has him shot down by guards. By this time, however, the Doctor, aided by a band of human resistance fighters who have shaken off the Aliens' control, has already managed to put a stop to the war games. Unfortunately he has no way of returning all the human soldiers to their proper times and places and so has to call on the Time Lords for help.

Having thus revealed his position to them, he is taken prisoner, placed on trial for the crime of interfering in the affairs of other races and subsequently sentenced to a period of exile on Earth. Jamie and Zoe, meanwhile, are sent back to their own respective points of origin. Before being dispatched to begin his exile, the Doctor is told that he must again take on a new appearance...

32 out of 121 (26.4%) raters say this story requires a previous story.