When barber's assistant Wilbur (Jerry Lewis) unwittingly becomes involved in a heist, he tries to get away by train. He is unaware that gangster Noonan (Raymond Burr) has slipped a stolen diamond into his pocket. Unable to pay the fare, Wilbur dresses up as a young boy and sits with teacher Nancy (Diana Lynn). When one of Nancy's colleagues sees Wilbur, she concludes that Nancy is cheating on Bob (Dean Martin), her fiancé. Wilbur remains disguised to prove Nancy's innocence and evade Noonan.