The doctor is a 906 year old time lord from the planet galefray. in the doctors first form( played by wiliam hartwell) he stole a tardis ( Time And Relative Dimention In Space ) because he got board of his home planet and wanted to explore the wonders of the universe.
After a battle with the cyber-men the doctor regenerated into a much diferent man( played by patric thortan). But in his final adventure the war games he was exiled to earth and forced to regenerate by the time lords for stealing the tardis on earth the third doctor ( jon pietwee ) became became u.n.i.ts scientific advisor.
The third doctor renerated when his body was damaged by radiation into a man much younger than the other three doctors butthe forth doctor ( Tom backer ) was able to travel in the tardis once again.
The forth doctor unfortunatley fell of a radio tellicope and regenerated into the fith doctor ( peter davison ) who for some reason wore a peice of celery on his jacet but he regenerated because of a deadly poision this time he regenerated intoa much more stricter and bosier doctor who wore awful cloths (played by colin baker).
But yet again the doctor regenerated this time the tardis was shoot down by the rani who is a rivil time lord when the the tardis crash landed he regenerated into he seventh form (sylvester mcoy) who was slightly different