Hard to Be a God.

The distant future. Earthlings help less developed civilizations overcome difficult stages of evolution. Anton, an employee of the Institute of History, goes to the medieval Arkanar to rescue scientists who can advance evolution. He will become Don Rumata of Estor, a wealthy aristocrat and lover of fights, booze and women. The main rule of progressors is not to interfere directly with the natural course of history, pointwise saving a few enlighteners from death. But Anton has a personal interest that he hid from the institute: somewhere in Arkanar, his father went missing.