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|Bud Abbott||Bud Alexander|
|Lou Costello||Lou Francis|
|Nancy Guild||Helen Gray|
|Arthur Franz||Tommy Nelson|
|Adele Jergens||Boots Marsden|
|Sheldon Leonard||Boots Morgan|
|William Frawley||Det. Roberts|
|Frederic I. Rinaldo|
Lou Francis (Lou Costello) and Bud Alexander (Bud Abbott) have just graduated from a private detective school. Tommy Nelson (Arthur Franz), 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 (Nancy Guild). He wants her uncle, Dr. Philip Gray (Gavin Muir), to inject him with a special serum he has developed which will render Tommy invisible, and hopes to use the newfound invisibility to investigate his manager's murder and proved his innocence. Dr. Gray adamantly refuses, arguing that the serum is still unstable, but as the police arrive Tommy injects himself with it and successfully becomes invisible. Detective Roberts (William Frawley) 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 (John Day), out cold. Morgan (Sheldon Leonard), 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 Hanlon. Tommy, still invisible, gets into the ring with them and again knocks out Hanlon with the illusion that 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), poor Hanlon is knocked out yet again. Morgan plans Bud's murder which is thwarted by Tommy, who unfortunately 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. Unfortunately, some of Tommy's blood has apparently entered Lou,who briefly turns invisible, only to reappear with his legs inexplicably on backwards.