Frank (Rainn Wilson) is an average Joe who works at a diner. When his wife Libby (Liv Tyler) leaves him he's understandably upset, so he goes to the police and reports it. They can't help him because she left him of her own free will. He simply doesn't understand this. So he decides to take things into his own hands to get her back, which culminates in him assuming the superhero persona of The Crimson Bolt. What prompts him to do this is a horrible Christian TV show called The Holy Avenger. In these TV clips director James Gunn portrays a tongue wagging Satan. which is utterly hilarious.
Frank believes that his hallucinations are real and God is telling him what to do. Since he doesn't have any super powers he uses a big wrench to literally clobber people. Many local drug dealers are put into the hospital, though he also beats someone who cuts in line at a movie theater. No bad-doer is safe! Along the way he picks up a sidekick, Bolty (a hilarious Ellen Page), who takes this all too seriously and even wants to kill people without a second thought. She's also sexually interested in The Crimson Bolt but Frank keeps on putting her off since he's still married and in love with his wife.
In the end there's a big showdown with the villain, Jacques (Kevin Bacon), the drug dealer who has Libby. In a series of flashbacks we see that Libby was a drug addict going to meetings and had met Frank when she was not using. Frank comes prepared, though, to take down Jacque's henchmen with everything from pipe bombs to projectile daggers. He even makes up a pair of Wolverine-like claws for Bolty.
SUPER is one of the best "realistic Superhero" movies out there. I liked it far more than I enjoyed the movie of KICK-ASS, primarily because there's much more heart to it.