Millennium

Extraterrestrial explorers to a planet (everyone seems to be human, no one seems to be from Earth) left behind quite a lot of their equipment when they left a few hundred years ago. Now they've come back to fetch it, but the people who found the stuff (telepathy rings, force field belts) have set themselves up as aristocracy on the feudal planet.
There's a nice second plot of a ruthless serf who kills his master, steals the trappings of power, and runs to another country to pass as an aristocrat.