Jack Brown arrives home at 4am after another night out drinking with his pals. His father, a police chief, reprimands him for being an idler and wasting his life. He is not amused when Jack announces that he really wants to become a policeman, and tells his son that he is quite unsuitable.
But Jack is not so easily dissuaded. After a medical examination and some basic training, he becomes a bobby on the beat. He keeps his new job a secret from his father, and pretends to go fishing when he is called for police duties, hiding his uniform underneath his fishing clothes.
Jack is attracted to Ivy, a young woman he meets on duty one day. He follows her to the Café Loch Lomond, run by her old nurse, Mrs. Bobday, who pretends to be Scottish and serves very bad haggis. Ivy has already left but Jack and Mrs. Bobday strike up a friendship.
Later that evening, Jack meets Ivy and Mrs. Bobday again - at a party given by his father for Ivy's birthday. He wants Jack to meet her. Ivy is confused - wasn't Jack the policeman she had met earlier? He tells the women that he doesn't want his father to know that he is Constable Brown.
Jack is introduced to Ivy's friend, Jules Martin. Jack and Mrs. Bobday take an instant dislike to the man. Jack and Ivy are getting better acquainted when the clock strikes, and just like Cinderella, Jack has to leave - changing into his 'fishing' clothes.
Jules Martin is really a crook, and is planning his next robbery, on Varnier, the jeweller. Jack is on traffic duty when he sees the 'smash and grab' on the shop, and he chases the thieves to Madame Tussaud's. He 'arrests' a waxwork dummy by mistake and is severely reprimanded by his astonished father, who suspends him from duty.
Mrs. Bobday is convinced that Jules Martin is responsible for the robbery. She and Jack disguise themselves, as a cleaner and piano-tuner respectively, and break into Martin's flat to look for evidence. The thieves discover them, but they manage to escape. Meeting Ivy later, Jack confronts her about Martin and tells her she must give him up, but Ivy storms off angrily.
Mrs. Bobday and Jack realize that the stolen jewellery must still be hidden somewhere at Madame Tussaud's. They go there to lie in wait and photograph the thieves when they return for it. They hide among the dummies until closing time, but when the thieves appear, the camera fails to work. Jack, pretending to be the dummy of an archer, accidently looses an arrow and startles the crooks. He and Mrs. Bobday hurriedly disguise themselves as members of various historical tableaux. The camera suddenly flashes, and they are caught by the thieves, but the police arrive just in time. Jewels and thieves are recovered. Jack is a hero, but back on point duty again. Ivy makes it up with him but their embrace holds up the traffic.
