Cliff Robertson plays Tolly Devlin, an embittered ex-convict who has spent a lifetime tracking down the men who murdered his father. Desirous of handling matters on his own, Devlin pretends to be loyal to both the Mob and the Government, playing one against the other in hopes of flushing out the killers. He learns that the three surviving assassins are employed by a supposedly charitable "cover" operation known as National Projects. To get what he wants, Devlin ingratiates himself with mob boss (and outwardly solid citizen) Conners (Robert Emhardt). What Robertson didn't count on was falling in love with "Cuddles" (Dolores Dorn), which leads to his own downfall -- but not before justice is served. Producer/director/writer Fuller based Underworld USA on a series of "exposé" articles in The Saturday Evening Post; the film's release fortuitously occurred shortly after that infamous mob convention in Appalachin, New York.