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Bud Alexander and his bumbling pal, Lou Francis, graduate from Dugan's Detective School and are assigned to the McQuilan Detective Agency. On their first night at work, champion boxer Tommy Nelson, who is wanted for the murder of his manager, O'Hara, rushes in and hires Bud and Lou to clear him of the charges. They follow him to the home of his girl friend, Helen Gray, whose uncle, Dr. Philip Gray, demonstrates a new invisibility serum on a guinea pig, which disappears before their eyes. Although Tommy wants an injection right away so he can elude the police, Phil explains that the serum turned its inventor into a raving, homicidal maniac, and that since he has no antidote, it is not yet safe for humans. Just then, however, the police arrive, and Tommy takes the serum as soon as Phil's back is turned and disappears in front of Lou. The police question Lou about Tommy's location, but when he tells them Tommy is invisible, they bring him to a psychiatrist to be hypnotized. Soon, Lou unwittingly hypnotizes the doctor and his entire staff, and is kicked out of the building. The next day, Bud, Lou and an invisible Tommy visit his old gym, where he points out his last opponent, Rocky Hanlon, and Rocky's financier, gangster Morgan. He explains that Morgan and O'Hara made a deal that Tommy would take a dive in his fight with Rocky, but when Tommy knocked Rocky out, Morgan killed O'Hara and framed Tommy. Tommy outlines his plan: Bud will pretend to manage Lou, a champion fighter, and Lou will agree to throw a fight with Rocky. When Lou then reneges, Tommy explains, and Morgan attempts to murder Bud, they can turn the mobster into the police. With Tommy doing the actual punching, Lou puts on a boxing exhibition and is promptly nicknamed "Louie the Looper." Morgan challenges him to fight Rocky, and though Lou is terrified at the prospect, Tommy promises he will be in the ring with him the whole time. At dinner that night, Morgan's moll, Boots Marsden, flirts with Lou, and he visits her hotel room later that evening with a recorder hidden in a huge bouquet of flowers. Lou manages to tape her encouraging him to throw the fight, but after he slips the recording into his pants, he immediately sits on it and breaks it. Meanwhile, the still invisible Tommy gets drunk in the restaurant downstairs and raves that he is all-powerful and will destroy anyone who gets in his way. Tommy provokes a fight with a stranger, and when the other man swings at Lou, Tommy is knocked out. Bud and Lou then attract attention when they drag him to Helen's home. There, Phil secretly straps Tommy down and holds him prisoner. Upon waking, Tommy accuses Phil of wanting only to protect his own professional reputation. Before the fight the next day, Bud and Lou wait nervously for Tommy to arrive. At the last minute, the boxer manages to escape his bonds and enter the ring, where he helps Lou pummel Rocky. When Tommy leaves the ring to trail Morgan, Rocky recovers and beats up Lou. Tommy jumps back into the ring and knocks Rocky out, but by the time Phil arrives at the fight with the invisibility antidote, no one can find him. Morgan plants one of his goons dressed as a policeman outside Lou's dressing room and then hides inside. While the henchman keeps the police out of the room, Morgan attacks Bud and Lou. Just then, Tommy arrives and grabs Morgan long enough for Lou to get the gangster's gun. Lou accidentally shoots the heating unit, and the escaping steam exposes Tommy's ghostly outline, allowing Morgan to stab him in the chest. As Tommy collapses, the police break down the door and arrest Morgan. Later, at the hospital, Tommy survives thanks to a blood transfusion from Lou. Lou receives a small amount of Tommy's blood, however, and turns invisible just long enough to wreak havoc by kissing all the nurses on the ward.