Hugo Z. Hackenbush is a veterinarian illegally employed as the medical director of the Standish Sanitarium, which is owned by Judy Standish. One of things they have to do to save the sanitarium from developers is to keep Mrs. Upjohn as a patient. She, of course, insists on being treated only by Dr. Hackenbush. To try to expose Groucho as a fraud, the bad guys call in Dr. Steinberg.
The film uses this plot as the framework around which to organize a series of skits. Among them is the "Tutsi Fruitsy Ice Cream" skit, in which Tony gives Hackenbush a tip on a horse, but all in code, so that Hackenbush has to buy book after book from Tony to decipher the code.
Another skit involves Tony and Stuffy trying to interrupt a frame job involving Hackenbush's seduction by a femme fatale. In the end, failing to dissuade Hackenbush from his interest in the woman, they end up disrupting the frame-up by concealing themselves under layers of wallpaper, using a bucket perched on Stuffy's head to hold the paste.
The overall plot involves Tony and Stuffy's friend, Gil Stuart's, difficulties with his racehorse, Hi-Hat, who seems hopeless as a racer.
After the police eventually come after the gang, Stuffy and Hi-Hat make their escape as the horse bounds over various obstacles with ease. At this sight, Gil realizes that Hi-Hat is actually an excellent steeplechase horse and enters him into the appropriate race. Unfortunately, the villains attempt to keep him out of the race and the gang must resort to various wacky tricks to buy time to get the horse into competition. In the race itself, the gang remembers that Hi-Hat is afraid of one of the villains and they pull more tricks to make the villain excited enough to yell in anger and inspire the horse to increase his speed. Eventually, there is a spill at the water obstacle and Stuffy and another jockey have to remount and Stuffy appears to lose the race. However, Stuffy realizes that he was riding the other jockey's horse and the judges correct the decision and Hi-Hat is declared the winner.
The screenplay went through numerous outlines, treatments and drafts before arriving at its final version. As they had with A Night At The Opera, the Brothers honed the comic material during a Vaudeville tour.
Groucho's character was originally named "Quackenbush" but was changed to "Hackenbush" over fear of a lawsuit by a real Dr. Quackenbush.
During production, Irvine Thalberg, who had brought the Marx Brothers to MGM, died suddenly of pneumonia at age 37. It is generally believed that the studio never gave the proper care after Thalberg's death to the Marx Brothers and that the three movies made at MGM afterward are weaker than the first two.