The origins of the sixth regeneration into the Seventh Doctor (played by Sylvester McCoy) is shrouded in mystery. What is known is that after the TARDIS was attacked by the Rani it was forced to land on the planet Lakertya.
What is not known is how the Doctor became injured severely enough to bring about regeneration. It has been suggested that this regeneration may have been intentionally forced by the Sixth Doctor in order to prevent chains of events that lead to the creation of the Valeyard.
Following this regeneration there was the now usual period of personality adjustment, in this incarnation resulting in a Chaotic, almost comic character. Included in this period of adjustment was amnesia which caused him to briefly mistake the Rani for his companion Mel.
While still under the Rani's influence, the Doctor chose a new look for his persona, shedding the chaotic, clown-like attire of his predecessor for a more subtle suit and hat. After regaining his memory and defeating the Rani, the new Doctor continued his travels with Mel by his side.
Eventually Mel chose to leave the doctor and the Doctor met Ace, a troubled teenager from 1980's earth, somehow transported to the planet Iceworld.
The old enemies the Daleks and the Cybermen both make appearances, as does Davros and the Doctor shows a darker side when he uses "The Hand" to apparantly destroy the Daleks homeworld, Skaro. UNIT and the Brigadier also make a welcome return. Later in the series the Doctor encounters his Arch Nemesis the Master, who is finally defeated.
Although percieved as a clownish figure, this personality is much darker, more manipulative and often looking at the bigger picture but not eleborating on this to his companions.
This Seventh incarnation ends when he is mistakenly transported to pre millenium San Francisco and is shot in a Gangland gunbattle. He dies in hospital and does not regenerate into the Eighth Doctor for several hours due to being under anaesthesia at the time of death.
Better to remain silent and be thought a fool than to speak out and remove all doubt.