The story follows an unnamed narrator who visits a mental institution in southern France (more accurately, a "Maison de Santé") known for a revolutionary new method of treating mental illnesses called the "system of soothing". A companion with whom he is travelling knows Monsieur Maillard, the originator of the system, and makes introductions before leaving the narrator. The narrator is shocked to learn that the "system of soothing" has recently been abandoned. He questions this, as he has heard of its success and popularity, but Maillard tells him to "believe nothing you hear, and only one half that you see".
The narrator tours the grounds of the hospital and is invited to dinner, where he is joined by twenty-five to thirty other people and a large, lavish spread of food. The other guests are dressed somewhat oddly: though their clothes are well-made, they do not seem to fit the people very well. Most of them are female and are "bedecked with a profusion of jewelry, such as rings, bracelets and earrings, and wore their bosoms and arms shamefully bare". The table and the room are decorated with an excess of lit candles wherever it is possible to find a place for them. Dinner is also accompanied by musicians playing "fiddles, fifes, trombones and a drum", and though they seem to entertain all the others present, the narrator likens the music to horrible noises (at one point even mentioning the torture and execution device known as the brazen bull). The narrator says that there is much of the "bizarre" about everything at the dinner.
Conversation as they eat focuses on the patients they have been treating. They demonstrate for the narrator the strange behavior they have witnessed, including patients who thought themselves a teapot, a donkey, cheese, champagne, a frog, snuff tobacco, a pumpkin, and others. Maillard occasionally tries to calm them down, and the narrator seems very concerned by their behavior and passionate imitations.
He then learns that this staff has replaced the system of soothing with a much stricter system, which Maillard says is based on the work of a "Doctor Tarr" and a "Professor Fether". The narrator says he is not familiar with their work, to the astonishment of the others. It is finally explained why the previous system was abandoned: one "singular" incident, Maillard says, occurred when the patients, granted a large amount of liberty around the house, overthrew their doctors and nurses, usurped their positions, and locked them up as lunatics. These lunatics were led by a man who claimed to have invented a better method of treating mental illness, and who allowed no visitors except for "a very stupid-looking young gentleman of whom he had no reason to be afraid". The narrator asks how the hospital staff rebelled and returned things to order. Just then, loud noises are heard and the hospital staff breaks from their confines. It is revealed that the dinner guests are, in fact, the patients, who have just recently taken over. As part of their uprising, the inmates treated the staff to tarring and feathering. The keepers now put the real patients, including Monsieur Maillard (who had once been the superintendent before going mad himself), back in their cells, while the narrator admits that he has yet to find any of the works of Dr. "Tarr" and Professor "Fether".
