The film depicts the misadventures of Lou Francis and Bud Alexander, two private detectives investigating the murder of a boxing promoter. The film was part of a series in which the duo meet classic characters from Universal's stable, including Frankenstein, the Mummy and the Keystone Kops.
Lou Francis and Bud Alexander have just graduated from a private detective school. Tommy Nelson, a middleweight boxer, comes to them with their first case. Tommy recently escaped from jail after being accused of murdering his manager, and asks the duo to accompany him on a visit to his fiancée, Helen Gray. He wants her uncle, Dr. Philip Gray, to inject him with a special serum which will render Tommy invisible, and hopes to use the newfound invisibility to investigate his manager's murder and prove his innocence. Dr. Gray adamantly refuses, arguing that the serum is still unstable, recalling that the formula's discoverer, Jack Griffin, was driven insane by the formula and did not become visible again until after he was killed. However, as the police arrive Tommy injects himself with it and successfully becomes invisible. Detective Roberts questions Dr. Gray and Helen while Bud and Lou search for Tommy.
Helen and Tommy convince Bud and Lou to help them seek the real killer, after Tommy explains that the motive for the murder occurred after he refused to "throw" a fight, knocking his opponent, Rocky Hanlon, out cold. Morgan, the promoter who fixed the fight, ordered Tommy's manager beaten to death while framing Tommy for the crime. In order to investigate undercover, Lou poses as a boxer, with Bud as his manager. They go to Stillwell's gym, where Lou gets in the ring with Rocky. Tommy, still invisible, gets into the ring with them and again knocks out Hanlon, making it look like Lou did it, and an official match is arranged. Morgan urges Lou to throw the fight, but when the match occurs (with the aid of an invisible Tommy), Hanlon is knocked out yet again. Morgan plans Bud's murder, but is thwarted by Tommy, who is wounded in the battle and begins to bleed badly. The protagonists rush to the hospital where a blood transfusion is arranged between Lou and Tommy. During the transfusion Tommy becomes visible again – some of Tommy's blood has apparently entered Lou, who briefly turns invisible, only to reappear with his legs inexplicably on backwards.