Doctor Who: Why Each Doctor Was Forced To Regenerate

After an incredible seven-year run as the Doctor - spanning from 1974's Robot to 1981's Logopolis - it was time for Tom Baker's Fourth Doctor to say goodbye, and his regeneration... well, it was a bit of a weird one.

Throughout Logopolis, a strange, pale figure can be seen lurking in the background of certain scenes, and initially, it's unclear what its purpose is. It's just this creepy unexplained alien thing, but all is made clear at the tail-end of the serial.

While attempting to stop the Master's latest evil plan, the Doctor falls from a radio tower, sustaining injuries that he cannot recover from. Here, the Watcher appears once more, wandering over to the Doctor and merging their two bodies together: the Watcher had been some sort of odd manifestation of the Doctor the entire time.

This merge helps the Doctor regenerate, and a few seconds later, the Fifth Doctor is born, with a rather cheeky grin on his fresh, youthful face.