It’s that old, familiar story: some scientists at a megacorporation unintentionally open a dimensional rift. Flash forward and an alien race has used the rift to conquer Earth. You must defend it with your crowbar. Or maybe you’d prefer being on a planet that’s really a machine built by an alien race at war with humanity (oh, and there’s this other alien race that happens to be parasitic) with a hologram in your head.
You know what? Sometimes the simplest concepts are best. At the end of a long day, you don’t always need to know what your motivation is for doing something. Sometimes you just need “Eat good things to win; Eat bad things and die.”