Usando sua paixão por histórias em quadrinhos, o adolescente Dave Lizewski decide se reinventar como super-herói, apesar da total falta de poderes especiais. O adolescente arruma uma fantasia, batiza-se de "Kick-Ass" e começa a combater o crime com a ajuda de amigos justiceiros.

After painter Michael 'Mike' Sheldrake's failed suicide attempt, house-mate and life-long best friend Peter Tremaine, an antiques shop owner, reminisces their common past, like Mike does in therapy. It started at the beach, when scrubby, scared Mikey become the dependent protégé of forceful Pete, both doted by Pete's dad, a wealthy lawyer. Afterwards Mike moves out to marry Kate. Pete accepts to be best man, but overdoes that, 'arranging' to get stuck in the honeymoon hotel he paid for. Drama from their boarding school childhood intertwines with tragedy for the mates, Kate and vindictive former police detective Miller.