While the Bowery Boys, Horace Debussy "Sach" Jones, Jr., Junior, Chuck, Butch and their leader, Terence Aloysius "Slip" Mahoney, watch a Marines propaganda film at Louie's soda shop in the Bowery district of New York City, Slip receives a draft notice. Within days he is hard at work in boot camp and discovers that the rest of the boys have been drafted. Tracing the trail of havoc that Sach has wreaked across the camp, Slip finds his bumbling pal impersonating a doctor and examining Col. Thomas Brown. For this offense, the boys are put on K.P. duty, and there Sach fills the cook's soup with huge amounts of hot sauces and peppers. When Capt. Miller tastes it and becomes sick, Sach is sent to Brown's office, but once there, the colonel receives word that Sach's soup can be used as a weapon. Brown congratulates Sach and then, upon realizing that Sach's father was his drill sergeant and good friend, promotes Sach to sergeant. Sach's new position immediately goes to his head, and he begins to lead his men, who include the Bowery Boys, on long, hard hikes. On one hike, Slip discovers Pvt. Hardy lying near a stream, beaten unconscious, and after calling an ambulance, picks up a playing card lying near the body. The next day, Sach procures five twelve-hour passes and takes the bys out on the town. On the street, he woos beautiful Lulu Mae, who takes them all to a gambling house run by Jolly Joe Johnson. The boys watch in wonder as each of their successful wagers suddenly becomes a losing hand before their eyes. Slip, realizing the tables are all rigged, notices that the blackjack cards match the design of Pvt. Hardy's card. Later that night, after Sach loses all his money, the boys plot to have him demoted. Slip places a fake bomb in Sach's arms as he sleeps and when the sergeant wakes, he disrupts the entire camp by racing around screaming that the enemy has attacked. Miller chastises Sach but, when he throws the bomb out of the window, it explodes, and as a result Sach is promoted to staff sergeant and awarded a medal. The next day, Slip learns that Hardy has died and so visits the local sheriff and conjectures that Hardy was killed when he discovered Johnson's outfit was crooked. The sheriff, who is on Johnson's payroll, informs Johnson immediately and they decree that Slip must be quieted. Meanwhile, Slip and Sach also go to Brown, who puts two intelligence officers, Maj. Desmond and Capt. Harlow, on the case. Together, they all go to Johnson's to witness the crooked gambling, but Johnson recognizes them and allows them to win. On bivouac the next day, Slip camouflages his foxholes so cleverly that first Miller and then Brown fall in, and Sach is promoted to technical sergeant for supposedly teaching his men so well. Sach later loses his whistle and tears the boys' bunks apart looking for it. When an inspection is called, the boys are punished for their sloppy beds. A dance is held that evening, and Johnson sends Lulu Mae there to cause trouble for Sach. She sneaks into the barracks late that night and hides in his bed, but when Brown hears a woman is in the camp and searches for her, she hides from him and then lets the boys into Johnson's closed gambling house. Slip searches the tables and discovers that each has a set of magnets underneath to move the chips and cards around. After they prove Johnson is corrupt, the boys are sent back to the camp. There, they discover that Brown has been replaced by a new colonel, who immediately demotes Sach to private and promotes Slip to sergeant.