Abbott and Costello Meet the Invisible Man

Lou Francis and Bud Alexander are aspiring private detectives who have just graduated from a special school. Boxer Tommy Nelson becomes their first client. He recently escaped from prison, where he was put on false charges of murdering his manager. Nelson asks his friends to take him to his fiancee, Helen. The fact is that her uncle, Dr. Philip Gray, knows the secret of the "invisibility serum", which the boxer wants to inject himself in order to find the real killer and prove his innocence. The doctor categorically refuses, as the drug is too poorly studied and unstable. The police arrive at the house, and Nelson voluntarily gives himself an injection. Police officer Roberts interrogates the doctor and his niece, while Lou and Bud try to find the boxer who has become invisible.

Helen and Tommy persuade Lou and Bud to help them find the real killer. The boxer says that he suspects the organizer of his last fight, Boots Morgan, who pressured the late manager to have Nelson "leak" the fight. The investigation begins with a change of disguises: Lou poses as a boxer, and Bud is his manager. They meet in the gym with Rocky Hanlon, Tommy's latest rival. Lou knocks Rocky out with the help of an invisible Nelson. Morgan decides to organize an official duel and asks Lou to "merge" the match. However, in the ring, Hanlon is knocked out again, and again thanks to the invisible Nelson. Morgan is going to kill Lou for disobedience, but Tommy prevents this, although he himself is wounded in the fight.

The friends rush to the hospital, where Tommy is transfused with Lou's blood. As a result of this procedure, Nelson becomes visible again, and Lou, on the contrary, invisible - fortunately, only partially and for a short time.