The Marx brothers try to save a sanitarium by treating a rich woman with a veterinarian and by winning a horse race.
Judy Standish (Maureen O'Sullivan) needs money for her sanitarium; but boyfriend Gil Stewart (Allan Jones) tells her he bought a race horse. Rich Emily Upjohn (Margaret Dumont) is going to Dr. Hackenbush, but Tony (Chico Marx) says he is coming and sends a telegram to veterinarian Hugo A. Hackenbush (Groucho Marx). Morgan (Douglass Dumbrille) offers Judy $5,000 for the sanitarium. Hackenbush arrives, and Emily says she will pay his salary. Hackenbush insults business manager Whitmore (Leonard Ceeley), gives Emily a horse pill, and goes to the race track. Morgan fires jockey Stuffy (Harpo Marx), and Gil can't pay his feed bill. Tony sells Hackenbush horse tips by code with extra charges for several books.
Hackenbush calls back Whitmore, pretending he is in Florida and distracting his investigation. Tony tells Stuffy to watch Whitmore and presents him as a patient to Hackenbush, who examines Stuffy and says he is moronic. Tony learns that Hackenbush is a horse doctor. At a water ballet Gil sings "On Blue Venetian Waters." Gil warns Judy about Hackenbush and admits his horse is not doing well. Hackenbush dances with blonde Flo Marlowe (Esther Muir) and Emily, inviting them to his room. Tony plays piano. Stuffy plays a heavy piece that breaks the piano; then he plays the strings like a harp. Flo tells Whitmore that he has a date to ruin Hackenbush, and without words Stuffy explains it to Tony. In his room Hackenbush gets ready and lets in Flo. Tony and Stuffy come in and sit on Flo. They leave but come back as detectives. Flo holds Hackenbush; but Tony and Stuffy return to wallpaper them, and Whitmore and Emily find only Hackenbush.
Emily apologizes to Hackenbush, who asks her to pay Judy's notes. Dr. Steinberg (Sid Ruman) questions Hackenbush and Emily. Hackenbush tries to leave, but Gil stops him. Whitmore asks Hackenbush to examine Emily. Tony and Stuffy come in as doctors, and they wash. Stuffy takes her purse instead of her pulse and rips the dress off a nurse. The mayhem ends after Stuffy turns on the sprinklers, and the three ride the horse out. Judy brings blankets to Gil, Hackenbush, Tony, and Stuffy and then cries. Gil sings "Tomorrow Is Another Day." Stuffy plays the flute, and blacks sing "All God's Chillun Got Rhythm" and dance. Whitmore says that Hackenbush is a horse doctor, and Morgan makes the horse crazy. Stuffy rides him and jumps over a car. Morgan tries to stop Hi-Hat from racing; but Tony lets cars park on the track, and Hackenbush is chased over them by police. After a delay Stuffy rides Hi-Hat, and Morgan's voice causes him to win.
The zany Marx brothers make this lively entertainment, making fun of doctors, institutions, a rich hypochondriac, horse tips, selling gimmicks, greedy businessmen, and horse racing.