Men in Black (movie)
One day, a flying saucer crashes into Earth. The 'Bug' that it contains is a powerful alien that takes the body of a human. It is searching for an energy source called "The Galaxy".  Agents J and K must stop the bug before it can escape. This they manage to do. At the end of the film, Agent K decides that he doesn't want to do this work anymore, and is treated by Agent J so that he won't remember the work he used to do.