Doctor Who: Why Each Doctor Was Forced To Regenerate

One of the coolest, most suave Doctors of all time, the Third Doctor spent the majority of his regeneration on Earth, due to that aforementioned exile. But of course, he still bumped into plenty of alien beasties over the course of his life.

Speaking of which, the Third Doctor's final battle was against a massive hairy spider. This particular spider - known as The Great One - desired a special blue crystal that would give it the power to take over the universe, and while attempting to persuade it to stop, the Doctor is exposed to a lethal dose of radiation.

Even though he's able to make it back to the TARDIS - and even though The Great One is ultimately defeated - the damage has already been done, and it's irreparable.

The Doctor gets lost in the time vortex before returning to Earth, and - ending as he started out - he once again collapses to the ground, before Pertwee's face slowly fades into Tom Baker's: the Fourth Doctor has arrived.