I need help: a good idea for a storyline?

For my ignment i need to design a video game (pretty cool ignment, eh?) and i need help with the storyline. The main characters are 5 robots, one girl four guys, (i have designed them already). It is obviously set in the future and have a very futuristic theme (hovercrafts, aliens, robots etc.) please give me an idea for a storyline! thanks.