The TARDIS materializes in what appears to be an Earth war zone.  It turns out that they are actually on a planet where a race known as the Aliens have gathered soldiers from different wars in Earth's history.  They plan to use the hardened survivors to take over the galaxy.

The Doctor quickly recognises the War Chief, and assistant to the ruling War Lord, as a member of his own race.

He is providing the Aliens with SIDRAT machines that work similarly to the TARDIS.  He plans to use the Aliens, however, to further his own goals.  When the War Lord has the War Chief shot when his plans are discovered.  The Doctor and a band of fighters are able to stop to the war games.  Since there is no way for the Doctor to return all of the soldiers, he calls on the Time Lords, his people, for help.

He is soon taken prisoner by the Time Lords now that his position is revealed.  He is put on trial for crimes of interfering in alien affairs sentenced to exile on Earth and the removal of time travel secrets from his memory.  Jamie and Zoe are sent back to their own respective points of origin and their memories of the Doctor and their travels are erased.  Before beginning his exile the Doctor is told by the tribunal that he is to be forcibly regenerated.  When he arrives on Earth he will look completely different.