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 (American actor)), 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 prove his innocence. Dr. Gray adamantly refuses, arguing that the serum is still unstable, recalling that the formula's discoverer, Griffin (The Invisible Man), 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 (William Frawley) questions Dr. Gray and Helen while Bud and Lou search for Tommy.