The film adapts the story of Mephistopheles and Doctor Faustus by having the main character Hendrik Höfgen abandon his conscience and continue to act and ingratiate himself with the Nazi Party and so keep and improve his job and social position.

The plot’s bitter irony is that the protagonist’s most fond dream is to play Mephisto – but in order to achieve this dream he in effect sells his soul, and realises too late that in reality he is Faustus; it is the Nazi leader having a major role in the film (modeled on Goering) who is the true Mephisto.

Both the film and Mann’s 1936 novel mirror the career of Mann’s brother-in-law, Gustaf Gründgens, who is considered by many to have supported the Nazi Party and abandoned his previous political views for personal gain rather than conscience. (Playing Mephisto was indeed the peak of Gründgens’ career, though in reality this was long after the fall of the Nazis.) However, Mann’s book is satirical, making Höfgen more a lampoon than a character in his own right, while the film offers a more realistic exploration of a flawed but recognisably human character.

via Wikipedia