The Doctor Summons the Time Lords
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 Doctor can't allow this to continue of course, and so after breaking the Alien's control over a band of human resistance fighters, the group heads to the War Room to stop the fighting. But the Doctor realises he might not be able to get all of the soldiers back to their own time on his own.
The Second Doctor Regenerates
Patrick Troughton's time as the Second Doctor was brought to an end when the Doctor was sentenced to exile on Earth and orderd to take on a new appearance by the Time Lords