Sixth Doctor

Classic Series
The Fifth Doctor regenerated after contracting the fatal disease spectrox toxaemia on Androzani Minor, having given the only antidote to save his companion Peri. He remarked in his final moments that the regeneration felt "different", and only an act of will spurred on by hallucinations saved him from death. From these troubled beginnings sprang the Sixth Doctor, who initially suffered from several psychotic symptoms in the period directly following the regeneration. Eventually he stablised as an outwardly arrogant buffoon, but he was just as capable of compassion and violence as his predecessor.

He travelled with Peri - and sometimes the Whifferdil shapeshifter, Frobisher - until he stumbled, alone, out of the TARDIS into a space station operated by the Time Lords. Suffering from amnesia, he found himself once again on trial for interfering in the affairs of the Universe. He elected to represent himself in the trial, for which he and his prosecutor, the Valeyard, would present episodes from his life as evidence via the Matrix. Through these he learned that Peri was dead, the subject of foul experiments in brain transplantation, and that he would in his future meet Melanie Bush, a computer programmer from Pease Pottage, Essex. It was eventually revealed that the Valeyard was an evil, possible future version of the Doctor, who had falsified the Matrix evidence - including Peri's fate - with the aid of the High Council, in order to cover up a conspiracy which the Doctor had unknowingly uncovered. In return for securing the Doctor's execution, the Valeyard would have stolen the Doctor's remaining regenerations, beoming a true Time Lord in his own right.

The Doctor defeated the Valeyard and departed with Mel, causing some confusion, as he hadn't met her yet in his own timeline. It is assumed he returned her to his future self, and later met her for the first time, travelling with her until the TARDIS was caught in the Rani's energy beams, crash-landing on Lakertya. This crash landing was apparently enough to spark his regeneration into the Seventh Doctor (though see below).

Expanded Universe
In the novels and audio adventures, it is assumed that the Sixth Doctor has a great many adventures and meets other companions between the end of his trial and his end on Lakertya. He initially spends time lamenting his fate and, fearful of becoming the Valeyard, resolves to become a hermit on the planet Torrok and avoid ever meeting Mel, changing his own future. He is caught up in events there, however, and travels again with a new companion, Grant Markham.

After parting from Grant on less than friendly terms, he travelled alone, meeting up briefly again with Frobisher, before eventually meeting Evelyn Smythe. He and Evelyn had a deep friendship, each helping the other heal their emotional problems. The Doctor eventually meets Mel on Earth and, when she stows away in the TARDIS, decides to accept the inevitable.

Several alternate explanations are also given for the Sixth Doctor's regeneration, which seemed unwarranted given the severity of the crash. One theory suggests that the Doctor subconsciously sacrificed his sixth self so he could be reborn as Time's Champion. The Sixth persona's anger over this is suggested as the psychological cause for the Valeyard's evil.