On an alien planet the Doctor uncovers a diabolic plot to conquer the universe, with brainwashed soldiers abducted from Earth forced to fight in simulated environments, reflecting the periods in history from whence they were taken. The alien's aim is to produce a super army from the survivors; to this end, they have been aided by a renegade from the Doctor's own race: the Time Lords.
Joining forces with rebel soldiers, who have broken their conditioning, the Doctor and his companions foil the plan and stop the fighting. But the Doctor admits he needs the help of the Time Lords to return the soldiers to their own times, but in asking risks capture for his own past crimes including the theft of the TARDIS. After sending the message he and his companions attempt to evade capture, but are caught.
Having returned the soldiers to Earth, the Time Lords erase Zoe and Jamie's memories of travelling with the Doctor, and return them to the point in time just before they entered the TARDIS. They then place the Doctor on trial for stealing the TARDIS and breaking the rule of non-interference. The Doctor presents a spirited defence citing his many battles against the evils of the universe. Accepting this defence the Time Lords announce his punishment is exile to Earth. In addition the operation of the TARDIS is wiped from his memory and his next regeneration is imposed.