Mission to the Unknown - S3-E2
Trivia: William Hartnell appears in the credits at the end of this Doctor Who story... despite the fact that he did not actually appear in this episode at any stage. He had a contract that he would be in the credits for all episodes, which ended after he left the show in "The Tenth Planet." Though he wasn't shown, he had to be credited.
Answer: TARDISes are generally available for properly authorised use on Gallifrey; they're not usually assigned to a particular Timelord on a long-term basis. The Doctor stole his when he left his homeworld.
Tailkinker ★