A murder witness masquerades as a twelve-year-old to escape the killers.

At a swank Los Angeles hotel, criminal Noonan, who has just committed murder while stealing the Majuba Diamond, deliberately bumps into Bob Miles, a girls' school instructor from Twin Falls, Oregon, and slips the diamond into Bob's coat pocket to avoid arrest. Bob, who is in Los Angeles trying to get reinstated in the Army, then visits the hotel's tailor, where Noonan attempts to switch coats with him. Bob stops him, however, just as the police show up, demanding to search Noonan, a known felon. Finding nothing, the police release Noonan, but remain suspicious. Bob, meanwhile, drops by the hotel barbershop and mistakes Wilbur Hoolick, the French barber's bumbling assistant, for the barber. Eager to prove himself, Wilbur, who has donned a wig and goatee and speaks with a phony French accent, prepares to cut Bob's hair. Bob is quickly upset by Wilbur's ineptitude and, as he is storming out, Noonan shows up and surreptitiously removes the diamond from Bob's coat pocket and sticks it in Wilbur's back pants pocket. Noonan then hires Wilbur to give his wife a scalp massage at their home. Having been alerted by her husband, Mrs. Noonan goes along with the ruse and dances flirtatiously with Wilbur in order to get the gem. Moments after she grabs what she presumes is the diamond, Noonan bursts in and pretends to be an enraged, jealous husband, threatening Wilbur with a knife. Terrified, Wilbur runs off, agreeing to take the next train to his home in Blitzen, Oregon. As soon as Mrs. Noonan realizes that she took Wilbur's bubblegum, Noonan heads for Union Station. There, after learning that he is thirty-one cents short for the ticket to Blitzen, Wilbur ambushes a tall twelve-year-old boy and steals his sailor suit and eyeglasses. Then, posing as a child, he gets unsuspecting Bob, who has come to say goodbye to his girl friend, fellow teacher Nancy Collins, to purchase a half-price ticket for him. Later, on the train, Noonan sits next to Wilbur, who nervously spills the contents of his suitcase. When he spies Wilbur's goatee and wig, Noonan deduces his identity and chases him. Wilbur takes refuge in Nancy's drawing room and convinces her that he is a scared boy, traveling alone. Nancy mothers Wilbur during a thunderstorm, and he becomes smitten. The next morning, at Mrs. Brendan's School for Girls, Bob learns that Nancy's train is stuck outside town and drives to meet her, accompanied by Gretchen, headmistress Elly Brendan's jealous daughter. In Nancy's drawing room, Gretchen discovers Wilbur and gleefully assumes the worst. After telling Bob, Gretchen rushes back to school to inform her mother and the board about Nancy's shocking behavior. As they are voting to fire Nancy, however, Bob arrives with Wilbur, still dressed in the sailor suit. Wilbur's childish antics persuade the board of Nancy's innocence, and Wilbur is about to depart when he spies Noonan lurking on the school grounds. Thus cornered, Wilbur accepts Gretchen's invitation to stay with her family, and by the time Wilbur arrives at the Brendan home, Gretchen's twelve-year-old brother Mike has already gone through his suitcase and discovered his secret. After Wilbur persuades him not to say anything, Mike, an aspiring geologist, asks to keep the interesting "rock" he found in Wilbur's pants. Oblivious, Wilbur agrees and changes into some of Mike's "cool" clothes. Wilbur then meets Skeets, one of the many amorous students who have been assigned to watch him. While touring the grounds with Skeets, Wilbur notices Noonan and begins to kiss Skeets to avoid detection. Nancy witnesses the kiss and in her classroom later, lectures Wilbur about the proper way for young boys to kiss girls. As Wilbur is eagerly demonstrating improper kissing techniques on Nancy, Bob walks in and grows suspicious. Bob tries to trick Wilbur into revealing he is a man, but is interrupted by the arrival of two police detectives. In private, the detectives tell Bob about Noonan's pursuit of Wilbur and instruct him not to reveal his impersonation. Although Bob consents, his jealousy of Wilbur intensifies. Later, at the Brendans', Mike steams open a letter to Gretchen, and Wilbur learns that she has used her friendship with a colonel's daughter to prevent Bob's reinstatement. Anxious to eliminate his competition, Wilbur calls Gretchen's friend and, imitating Gretchen's voice, tells her she now wants Bob to re-enlist. Wilbur then packs his suitcase and starts to sneak away when he is confronted by Noonan, who tears Wilbur's pants off, searching for the diamond. Fleeing in his underwear, Wilbur hides in Nancy's apartment and is almost caught by Bob. Later, at the annual spring dance, Wilbur has a romantic chat with Nancy and, convinced that she would love him if he were an adult, runs to change into a tuxedo. When he returns, however, he overhears Bob and Nancy professing their love and discussing marriage. Wilbur has little time to mourn, as Noonan appears and starts to choke him. Wilbur escapes, and the next day, Mike informs Wilbur that he knows about the diamond and murder, having read a newspaper article about the case. Still unaware, Wilbur teases Mike, talking to him in a Humphrey Bogart "tough guy" voice. Scared, Mike slips the diamond into Wilbur's pocket just as Wilbur is called to Mrs. Brendan's office. To Wilbur's horror, Noonan is there, posing as Wilbur's father. Noonan drags Wilbur out of the office and finds the diamond in his pocket. Before he can make off with it, Bob and the detectives show up and give chase. Noonan jumps in a speedboat, and Bob pursues him in another boat after Wilbur ends up on some water skis behind Noonan. Noonan finally crashes and is arrested. Later, Bob, who has received his reinstatement notice, says goodbye to Nancy at the train depot and is horrified to discover that Wilbur is working onboard as a barber.