Abbott and Costello portray private detectives in this fast-paced comic thriller. Recent detective school graduates Bud and Lou are minding their new business when accused killer Tommy Nelson (Arthur Franz) bursts into their office. Proclaiming his innocence, he convinces the two to get him to his beautiful fiancee Helen (Nancy Guild). Just as the police arrive to investigate, Tommy injects himself with invisibility serum being developed by Helen's uncle. Nelson convinces Lou to go undercover as a boxer to help flush out the real killer. With Nelson's unseen assistance, Costello reluctantly goes into the ring as "Lou the Looper" for one of the most hilarious fight sequences ever filmed.