My Brother is a Superhero

Luke's in his tree house with his older brother, Zack, but he needs a wee.
When he gets back, his undeserving, never-read-a-comic-in-his-life brother has been given superpowers and told to save not one, but two, universes. Luke is OUTRAGED! Zack doesn't even like LIKE comics! But there are worlds to save and SOMEONE has to make sure Zack does it properly. Especially as he won't even wear a cape...