A Biography of the Doctor
The First Doctor
The TARDIS arrives in 17th Century Cornwall, to the astonishment of Ben and Polly. The Doctor is soon kidnapped by pirates, after learning a cryptic clue to the location of hidden treasure. He escapes when the local Revenue Officer arrives.
(placement rating 4)
The TARDIS arrives on Mars, where the Doctor, Ben, and Polly go camping. They are forced to leave in a hurry when the TARDIS's lateral drift compensators cause the TARDIS to begin taking off. When the Doctor tries to return and clean up his mess, he arrives at Earth's south pole, rather than Mars'.
(As related in Short Trips and Side Steps: Please Shut the Gate.)
THE TENTH PLANET
The temperature on the TARDIS suddenly drops, as it arrives at a space tracking station at the South Pole, 1986. The Doctor, who comments that "this old body is wearing a bit thin", lets on that he knows that there will soon be some "visitors". The tracking station has been trying to talk down a space capsule, but the Doctor soon discovers that its problems are caused by a rogue planet, Mondas.
Mondas, once Earth's twin planet, is now home to the Cybermen, who have replaced most of their organic parts with machinery. They have also eliminated emotions, and work by pure logic. They plan to drain Earth's energy to replenish Mondas, and convert humanity into Cybermen. General Cutler, commanding the base, tries to use the powerful Z-bomb to destroy Mondas - clearly unhinged by the death of his son. Ben, Polly, and one of the scientists from the base prevent him from doing this, as the bomb would release colossal amounts of radiation on Earth. Soon afterwards, reinforcement Cybermen arrive at the base, kill Cutler, and take Polly hostage. They force the humans to move the Z-bomb to a point where it could destroy Earth, thus preventing Mondas from absorbing too much of Earth's energy. This is prevented when the humans realise that the Cybermen are vulnerable to radiation. Eventually, Mondas disintegrates, when it absorbs too much energy. The Cybermen then collapse and shrivel, as they were dependent on energy from Mondas.
During the course of these events, the Doctor had become progressively weaker. He was clearly aware that his body was in poor condition, and that he was close to regeneration. Staggering back to the TARDIS, he entered. Feeling exhausted, aware that the change was upon him, he collapsed by the console, just managing to reach up and open the doors to let Ben and Polly in. Then he collapsed, and the change came upon him.