A Day at the Races was the Marx Brothers ' follow-up to their incomparable A Night at the Opera . Groucho Marx is cast as Hugo Z. Hackenbush, a veterinarian who passes himself off as a human doctor when summoned by wealthy hypochondriac Emily Upjohn ( Margaret Dumont ) to take over the financially strapped Standish Sanitarium. Chico Marx plays the sanitarium's general factotum, who works without pay because he has a soft spot for its owner, lovely Judy Standish ( Maureen O'Sullivan ). Harpo Marx portrays a jockey at the local racetrack, constantly bullied by the evil Morgan ( Douglass Dumbrille ), who will take over the sanitarium if Judy can't pay its debts. After several side-splitting routines—Chico selling Groucho tips on the races, Chico and Harpo rescuing Groucho from the clutches of femme fatale Esther Muir , all three Marxes conducting a lunatic "examination" of Margaret Dumont—the fate of the sanitarium rests on a Big Race involving Hi-Hat, a horse belonging to the film's nominal hero, Allan Jones . Virtually everything that worked in "Opera" is trotted out again for "Races", including a hectic slapstick finale wherein the Marxes lay waste to a public event. What is missing here is inspiration; perhaps this is due to the fact that MGM producer Irving Thalberg , whose input was so essential to the success of "Opera", died during the filming of "Races". Even so, Day at the Races made more money than any other previous Marx Brothers film—the result being that MGM, in the spirit of "they loved it once", would continue recycling Races ' best bits for the studio's next three Marx vehicles. — Hal Erickson