Mourning his father Anton, Ivan Vanko watches Tony reveal himself as Iron Man on the news and builds his own arc reactor from his father's old Stark blueprints.
Vanko attacks Tony at the Monaco Grand Prix with electrified whips, then returns at the Expo in armor with an army of drones. Nick Fury explains that Howard Stark had Anton deported for trying to sell the reactor.
The villain is the Stark family's past coming back: the same reactor technology, from the partner Howard pushed out.
Source: Wikipedia: Iron Man 2, Plot