The fifth Doctor Who was played by Peter Davidson, chosen as a contrast to Tom Baker, but yet an actor firmly established in the public's mind (in part due to his role in All Creatures Great and Small).
The regeneration into the fifth Doctor was a problematic one, which nearly failed. The Doctor briefly took on aspects of all four previous Doctors before recovering in Castrovalva.
The series took a back to basics approach, with a more series feel, and many of the Doctors enemies were re-introduced including The Master, Cybermen, Omega, Sea Devils and the Silurians.
The Fifth Doctor is considered to be the most human of all the incarnations, less pretentious and selfish and far more reserved. He preferred to react to situations rather than initiate them and gained peoples trust by honestly proving himself.
This humanity made the doctor more prone to indecision and to occasionally panic under pressure. He also showed the greatest abhorrence to violence and needless pain or bloodshed, unable to execute Davros in cold blood.
The fifth Doctor died after being exposed to the drug Spectrox in it's raw toxic form while on the planet Androzani Minor. With only one antidote dose available, he sacrificed his own life so that his companion (Peri Brown) would be saved, and as he regenerated into the sixth Doctor, expressed doubt for the first time ever that regeneration may not be possible.
Two things are infinite; the universe and human stupidity; and i’m not sure about the universe