Les Cerisiers Sanatorium is a luxury and idyllic home for the mentally ill, presided over by respected psychiatrist Mathilde von Sahnd. The story is set in the drawing room which connects to three other rooms, each of which is the living quarters of a mentally ill resident patient. The three patients are all physicists and are allowed to use the drawing room, where sometimes they will be monitored by the female nurses who take care of them. Patient One is Herbert George Beutler. He is convinced that he is actually Sir Isaac Newton. Patient Two is Ernst Heinrich Ernesti who believes that he is Albert Einstein. Patient Three is Johann Wilhelm Mobius who believes he is visited on a regular basis by King Solomon. As the play begins, Ernesti has just killed one of the nurses and the room is crawling with policemen examining the crime scene. As they talk they reveal that this is not the first slaying at the sanatorium in the last three months; Beutler committed the first murder.
In Act Two the motive behind the killings is revealed, none of the men in the asylum is actually insane. They are all faking insanity and are in fact genius physicists whose discoveries have pushed the boundaries of what it is believed man is capable of discovering. Afraid of what other men could use his discoveries for, Mobius decided to feign madness so that he would be committed to a sanatorium and be able to protect his discoveries. Beutler and Ernesti are spies from two different countries, and they have infiltrated the sanatorium with the objective of stealing Mobius' documents and papers, and if possible, Mobius himself. Both spies have killed a nurse to protect their own identities and to support the notion that they are certifiably insane.
All three men come to reveal their secrets. Both spies try to convince Mobius to accompany him but it is in fact the genius of Mobius that he manages to convince the spies that the knowledge he has discovered is too dreadful for man to know, or to be trusted with, and since he recently burned all the paperwork pertaining to his discoveries they are wasting their time and effort anyway. After deliberating the three men agree that together they will protect mankind from what Mobius has discovered and they will live out the rest of their lives in the sanatorium in an effort to serve both humanity and physics.
However, their noble agreement is thwarted when it is revealed that Fraulein Doktor von Zahnd reveals to the men that she has been eavesdropping on their conversation from beginning to end. She also reveals that she has known Mobius' true identity for many years and has been secretly copying his research documents, and using his discoveries to construct a global empire of her own. She believes that King Solomon himself is speaking directly to her and he will be guiding her in the use of Mobius' discoveries so that she will become the most powerful person on earth. The play ends in uncertainty and foreboding. The three men have been outplayed and each speaks directly to the audience, explaining their plight, and apologizing for what von Zahnd is going to inflict upon humanity.