When the Doctor regenerates into his fourth body at UNIT HQ, the Brigadier summons Lieutenant Surgeon Harry Sullivan to look after him. After helping the Doctor defeat Kettlewell’s giant robot, and disbelieving that the TARDIS was a space-time ship, Harry was whisked away with the Doctor and Sarah. He saw the Sontarans, the birth of the Daleks and the Cyber assault on Voga. He left the TARDIS after the attempted Zygon invasion, when he was held prisoner and used as a template for their shape-shifting. Sarah later said of Harry that his work developing vaccines had saved thousands of lives.
Oh, come along now, Doctor. We're both reasonable men. Now, we both know that police boxes don't go careering around all over the place.