Sounds simple, but there's more: Astronauts Grant (Koenig) and Tanner (Campbell) are on a non-disctipt mission that takes them near the Moon. While nearing the dark side of the Moon they discover a GIANT derelict spaceship and are ordered to check it out. (How people couldn't see something that size from earth, we'll never know.) They bring back a small football-shaped thing and a skeleton (that happens to float by while Grant is outside the ship) back to Earth with them. Once actually on earth, the "football" cracks open and a small robot jumps out. It uses the materials around it (including that skeleton) to build itself into a huge robot. Then it wreaks havoc for about 9 mins before it's destroyed and then a recon mission to the Moon is planned. And if you think I'm giving a bunch of stuff away, that's just the first 20 mins of the movie.
Reasons to watch: For a painful movie, there's a lot of action making it an entertaining painful. There's some pretty neat effects. And Bruce Campbell in his prime!
Movies it likely inspired: Promethius. There's actually a scene where the astronauts enter a room, on the Moon, and take their helmets off. Just like a real astronaut would!