In early 20th-century New York, former heavyweight champion Roy "Slag" McGurk (Wallace Beery) considers himself the "King of the Bowery." The truth is, Slag is hired muscle for local saloonkeeper Mike Glenson (Edward Arnold) and a kept man, thanks to the generosity of local pawnshop proprietress (and sometime girlfriend) Mamie Steeple (Aline MacMahon). He can't dodge a pair of sucker punches thrown his way in the forms of his former boxing protege, now a street salvationist, Johnny Burden (a baby-faced Cameron Mitchell) and orphaned English lad Nipper (Dean Stockwell). As Slag tries to help Johnny romance Glenson's daughter (Dorothy Patrick) despite her father's ardent objections, Johnny encourages Slag to turn over a new leaf. That turn needs to happen quickly, though, because Child Services threatens to separate Slag and Nipper. But Slag's only ever lost one fight, and he doesn't intend to lose again!