Judy Standish's (Maureen O'Sullivan) sanitarium is under threat of being taken over by unscrupulous businessman J.D. Morgan (Douglass Dumbrille), who wants to demolish it and expand his real estate monopoly. The only hope for Judy is if she can keep as a permanent patient the wealthy hypochondriac Mrs. Upjohn (Margaret Dumont), who will only stay if the sanitarium hires her favourite physician, Dr. Hugo Z. Hackenbush (Groucho Marx). Unfortunately for Mrs. Upjohn, Hackenbush is actually a confidence-trickstering veterinarian, who has to avoid being caught out by either Morgan's cronies, the Florida Medical Board or Mrs. Upjohn.
Meanwhile, Chico Marx and Harpo Marx play a sanitarium worker and a jockey who have to save Hackenbush from a Femme Fatale employed by Morgan, while simultaneously trying to get the racehorse owned by Judy's fiancée Gil (Allan Jones) into race-winning condition.
In addition to the usual tropes exhibited by the Marx Brothers, this film provides examples of:
- Acrofatic: During the "All God's Children" number, one of the dancers is an overweight man who pulls off some pretty impressive moves, culminating in doing the splits.
- Accidental Misnaming: Chico keeps pronouncing Groucho's character Hackenbush's name as "Hackannapuss".
- Berserk Button: Hi-Hat the horse utterly detests the main villain. This is used to advantage during the race.
- Blackface: The boys, evading the law, momentarily don (very badly done) blackface to hide among a bunch of stable hands. Harpo only paints half his face.
- Boy Meets Girl:"I can't hide it any longer. I love you. It's the old story, boy meets girl — Romeo and Juliet — Minneapolis and St. Paul!"
- Call-Back: to Duck Soup during the barn musical number, with the line "All God's children got swing." Previously it was "All God's children got guns."
- Covered in Mud: Harpo on the good guy's horse and another guy on the bad guy's horse are neck and neck to win when they take a spill into a big mud hole. Hurrying back into the race, the other guy comes in first...but a quick wash-off reveals it's on the good guy's horse.
- Doctor, Doctor, Doctor: Played for laughs when Groucho introduces Chico and Harpo as Dr. Steinberg to the actual Dr. Steinberg:Whitmore: Dr. Hackenbush, tell me, who sent for these men?
Hackenbush: You don't have to send for them. You just rub a lamp and they appear.
Tony: My name is Steinberg.
Hackenbush: Dr. Steinberg, by a strange coincidence, this is another Dr. Steinberg. May I take my great friend and introduce my colleagues and good friends, another Dr. Steinberg. This is a Dr. Steinberg, Dr. Steinberg. Dr. Steinberg. And a Mrs. Steinberg. And Doctor, I'd like you to meet another Dr. Steinberg. And, eh... that's a, that's a Steinberg junior.
- End of an Age: This film, although still hailed as a classic, has been noted as repeating the basic plot of A Night at the Opera and it was produced around the death of the MGM executive Irving Thalberg. With that, the Brothers lost their strongest advocate and their streak of comedy film excellence came to an end.
- Fake Static: When Whitmore calls the Florida Medical Research Board looking for information on Dr. Hackenbush, Hackenbush pretends to be a Southern colonel answering the phone and pushing the buzzer:Whitmore: Col. Hawkins, did you get a wire from me regarding Dr. Hackenbush?
Hackenbush: [turns on the fan and holds a piece of paper next to it] I'm sorry, sir, but there's a hurricane blowing down here, and you'll have to talk a little louder! Whew, it certainly is the windiest day we ever did have. Woo, it certainly is windy!
Whitmore [raising his voice]: I want to know about Doctor Hackenbush! [Hackenbush pushes the dictograph buzzer, summoning Whitmore away from the phone; Whitmore answers] Yes?
Hackenbush: Whitmore, you'll have to cut out that squawking. The patients are all complaining.
Hackenbush [disguising his voice as Col. Hawkins]: And, eh, I hope, sir, that's the information that you require.
Whitmore: I'm sorry, Colonel, I didn't hear it. I was called to the dictograph.
Hackenbush [as the Colonel]: What was that you said, sir?
Whitmore [raising his voice]: I was called to the dictograph!
Hackenbush [touches the buzzer, Whitmore answers]: Whitmore, one more yelp out of you, and I'll have you bounced out of here!
Hackenbush [as Col. Hawkins]: And I trust, sir, that that answers your question.
Whitmore: I'm terribly sorry, Colonel, I didn't hear you.
Hackenbush [as Col. Hawkins]: I can't hear you, you'll have to talk a little louder.
- Grande Dame: Margaret Dumont again.
- Have a Gay Old Time: Groucho telling the female lead, "For you, I'd make love to a crocodile." note
- Have You Told Anyone Else?: When Tony and Stuffy find out Hackenbush's secret, they tell Gil, who responds this way out of concern.Gil: Does anyone else know about this?Tony: Only Hackannapuss, and he won't talk!Gil: It's bound to get out sooner or later.
- Herr Doktor: Dr. Leopold X. Steinberg, played by German-born actor Sig Ruman.
- Induced Hypochondria: The reason Hackenbush is so well trusted by Mrs Upjohn:Mrs Upjohn: Why, I never knew a thing was wrong with me before I met him.
- Lopsided Dichotomy:"Either this man is dead, or my watch has stopped".
- M.D. Envy: Groucho averts this by just practicing on humans anyway.
- Mysterious Middle Initial: Dr. Hugo Z. Hackenbush.
- Nonverbal Miscommunication: When Harpo tries to warn Chico about the frame job against Groucho. He gets it eventually.
- Only One Name: Tony and Stuffy.
- Parlor Games: An Overly-Long Gag has Stuffy spend an entire scene using charades to explain the villains' Frame-Up plot to Tony.
- Real Joke Name: Averted, Groucho Marx's character was going to be Doctor Quackenbush, but MGM's Legal department heard from an actual Dr. Quackenbush complaining, so the name was changed to Hackenbush.
- Sexy Figure Gesture: In the scene where Stuffy does a pantomime to tell Tony about The Vamp who plans to seduce and frame Dr. Hackenbush. When Stuffy makes the hourglass gesture, Tony's first guess is, "A snake! A big, big snake!" After Stuffy's gestures and whistles get more and more exaggerated, "That's-a no snake." (The Marx Brothers used a similar gag in their later movie Go West.)
- Suspiciously Similar Substitute: As with A Night at the Opera, Allan Jones fills in for Zeppo.
- This Means War!: This is Hackenbush's first reaction when he hears the bugle from the racetrack—it got him out of the heated discussion he was having with Whitmore.Hackenbush: So it's WAR! I'm off to the battlefield!Mrs. Upjohn: No no! That's from the racetrack.
- Wacky Americans Have Wacky Names: Dr. Hugo Z. Hackenbush.