Lou Francis and Bud Alexander are novice private investigators who have just graduated from a special school. Boxer Tommy Nelson becomes their first client. He has escaped from prison, where he was wrongfully convicted of murdering his manager. Nelson asks his friends to accompany him to his fiancée, Helen. The thing is, her uncle, Dr. Philip Gray, has invented an “invisibility serum” that the boxer wants to inject himself with so that, using his invisibility, he can find the real killer and prove his innocence.