It's important to know this going in, and it's also important to get some background on show as the game doesn't do a great job of filling in those blanks.
Ben 10, for those not familiar, is an action cartoon that runs on The Cartoon Network where a boy finds a wrist device that is attached permanently to his wrist allowing him to transform into 10 different alien forms, and like all cartoons, he ends up saving the world doing it.
