Doctor Who: Every Version Of The Doctor Explained
22. The Third Doctor (First Appearance: 1970)
The Doctor was exiled to Earth for much of his third incarnation, and became much more of an action hero than the two Doctors before him.
He was employed by the United Nations Intelligence Taskforce as a scientific advisor, although he often clashed with his boss, the stern yet respectable Brigadier Lethbridge-Stewart.
The Doctor and the Brigadier would become best of friends, and remained loyal to each other until the Brig's death in 2011. The same is true of Sarah Jane Smith, who first met the Doctor in his third incarnation and would become the most beloved companion in the history of the show.
One of the Doctor's other best friends/enemies, the Master, was also introduced during the Third Doctor era. The dynamic between Jon Pertwee's Doctor and Roger Delgado's Master has rarely been equalled in future incarnations.
The Third Doctor's life came to an end when he exposed himself to fatal levels of radiation to destroy an enormous spider known as The Great One. He regenerated on the floor of UNIT headquarters, as the Brigadier and Sarah Jane looked on.