Sixth Doctor (The People Made of Smoke)

The Sixth Doctor was the result of the regeneration of the Fifth Doctor, caused by the Smoke.

The Doctor believed that he would die by trapping himself in the TARDIS and allowing the Smoke to repeatedly kill him until he ran out of regenerations, but Abby and Zara restored him in his fifth incarnation when the Smoke died. (AUDIO: The People Made of Smoke)