Before it was a band in the Scott Pilgrim universe, it was an NES game. A fucking AWESOME game.
You know you've got a good game when instead of starting it, you ATTACK.
The game has you playing Billy "Big Bang" Blitz, a secret agent who, for some reason, looks like he's auditioning for My Chemical Romance. You're pulled back to duty from vacation to rescue Professor Plum, who's been captured by an evil gang called The Lawbreakers to build a Doomsday Bomb that threatens to blow the world up. What the gang planned to do after blowing literally everything up is not elaborated upon.
You chase the members of the gang and other assorted baddies through this map, (which might be set in North-Eastern Wyoming). As you go along, the plot spins completely out of control with a strange series of subplots involving demons and aliens. At one point you even talk to a dead guy.
We'd rather not give out spoilers, even for a 20+ year old game, but suffice it to say the ending is nuts and you still have a chance to fuck up and lose the game even at the last possible minute.
Or, you know, you could just track down a playthrough on Youtube. Your call.
In the Scott Pilgrim universe, The Clash At Demonhead is a band fronted by Scott's ex Envy Adams and including Todd Ingram, one of Ramona's Seven Evil Exes. The name of the third member, Lynette Guycot, is another reference to the NES game. In the movie, the band's music is performed by the band Metric.