At Louie Dumbrowski’s sweet shop in New York’s Bowery district, friends Terence Aloysius "Slip" Mahoney, Horace DeBussy "Sach" Jones and Butch Williams pay for their banana splits with yet another worthless check. Louie berates the boys for their lack of ambition and says they should be more like their other pal, Chuck Anderson, who works as a copy boy at the Blade newspaper. Just then, Chuck rushes in with news that he has been promoted to reporter and assigned to a big story, although he does not elaborate. Later, another Blade reporter named Sy Bowman comes to the shop and informs the friends that Chuck was badly beaten in a riot at the state penitentiary, where he was working undercover to investigate corruption. Horrified, Slip and the others immediately go to the hospital to visit Chuck, but before Bowman can leave, he is menaced by Willy, the thug who works for a bookie to whom Bowman owes $5,000. Bowman promises to pay soon, while at the hospital, a tearful Slip vows vengence when Chuck reveals that “Big Greenie” beat him. Bowman meets the boys at the hospital, and Slip decides to commit a real crime so that he, Sach and Butch will be sent to the penitentiary to find Chuck’s attacker and uncover the corruption. Bowman agrees to the plan, which will require him to notify the prison’s warden, Oswald, and the Blade ’s editor of the boys’s innocence, as well as hold the loot from their robbery for safekeeping. That night, the boys rob Potnick’s jewelry store and leave many clues as to their identities, including fingerprints and a stamp stating “Sach was here.” The police are baffled about the crime, however, and do not arrest Slip and the others until Bowman makes an anonymous call implicating them. Unknown to the boys, as they are entering the penitentiary, Bowman sells the goods from Potnick’s and uses the money to pay off his bookie. In the prison, the boys are taken by tough guard Jenkins to see Oswald, who has been the warden for only a few weeks. Oswald, who was not informed by Bowman about the undercover investigation, is puzzled by the boys’s insistence that they will be leaving in a few weeks, and orders Jenkins to keep a close eye on them. Slip and his pals then meet their cellmate, Hank Gaffney, who reveals that he has been digging an escape tunnel for twenty-two years. After many problems trying to adapt to the prison work routine, the boys are ordered by Jenkins to mop floors. The area they are taken to houses gangster Percival P. Lannigan and his henchmen, including Tom Tomcyk and Big Greenie. Slip, Sach and Butch are amazed by Lannigan’s lavishly appointed cell, as well as the gourmet food he is served and the luxurious clothes he wears instead of a uniform. They overhear him talk about the payoffs he makes to the guards, with Jenkins serving as his go-between, and watch as he slips a notebook filled with details of his payments into his desk. After the gangster orders Slip and Sach to clean his cell, they attempt to steal the notebook but fail. They then demand to see Oswald and tell him about their connection to Bowman. When Oswald calls Bowman, however, he denies knowing the boys. The warden then visits Lannigan’s cell, but it has been transformed to look like everyone else’s. Believing that the boys are insane, Oswald sends them to prison psychiatrist Fernando F. Fordyce, who is driven crazy by the boys’s constant bickering. Slip realizes that Bowman has double-crossed them, and when Louie visits, tells him that he must take Chuck to see the editor of the Blade to verify their story. Greenie overhears their conversation and reports to Lannigan, who orders his men “on the outside” to pick up Louie and Chuck. When the boys return to their cells later, they learn from Hank that Lannigan intends to have them killed the following day, and that he has captured Louie and Chuck. Desperate to save their friends, the boys try to escape using Hank’s tunnel, but it emerges in the warden’s office. Just then, Oswald enters, but he assures the boys that he now believes them, as he made a surprise inspection of Lannigan’s cell and found the luxury items they described. Before Oswald can alert the prison board, Lannigan, Tomcyk and Greenie arrive, and a brawl ensues. Through Sach’s trickery, Tomcyk and Greenie plunge down into Hank’s tunnel and are knocked unconscious, while Slip beats Lannigan into revealing where Louie and Chuck are being held. Slip gets Lannigan’s notebook, then tosses him into the tunnel. Oswald calls the police, who rescue Chuck and Louie, then apologizes to the boys for doubting them. He promises to arrange for their releases but urges them to be patient. Much later, Slip, Sach and Butch are still in their cell, remembering Oswald’s words about patience as they stroke their long beards.