The following version of this book was used to create this study guide: Pearse, Sarah. The Sanatorium. Viking, 2021.

The novel is narrated by an omniscient narrator in the present tense. In 2015, Daniel Lemaitre visits the luxury hotel he designed with his property developer friend, Lucas Caron. The hotel had once been a sanatorium, but they had reworked it into a modern masterpiece high up in the remote Swiss Alps. Daniel finds a bracelet in the parking lot and is then attacked by someone.

In 2018, Le Sommet opens to the public. In January of 2020, Elin travels to Le Sommet with her boyfriend, Will. Elin is a police detective in England who is on extended leave. Almost a year ago, she was attacked by a murderer and nearly killed. Ever since then, she has been experiencing panic attacks. She cannot stop thinking about her younger brother, Sam, and his untimely death when they were children. When her brother, Isaac, invites her to his engagement party at Le Sommet, she decides to go in order to figure out if he killed Sam, since she has always suspected this is true.

One of the maids at Le Sommet, Adele, finishes her shift and heads home to see her son, but she is attacked by someone in a mask. Adele runs through the woods, but her attacker corners her and ties her up in a shack in the woods. The captor shows her a photograph and then gives her a mask.

Elin and Will are greeted by Isaac and his fiancé, Laure, who Elin was childhood friends with. Laure shows Elin the archive room full of artifacts from when the hotel was a sanatorium. Elin makes plans to meet up with Isaac for dinner, but he does not show up. That night, Elin goes onto her balcony and hears Laure arguing with someone on her cellphone in French.

At breakfast the next morning. Isaac tells Elin that Laure is missing. Elin does not think much of it at first, but Isaac is convinced something terrible has happened to Laure. A skier finds the remains of Daniel Lemaitre buried in the snow. Laure goes to the spa and feels as if someone is watching her. She talks to Laure’s friend, Margot, who works in the spa. Margot tells Elin that Laure and Isaac have been having relationship problems. Elin searches Laure’s desk and finds phone bills for a secret phone and an article about psychotic depression. Isaac shows Elin a collection of photographs Laure had of Lucas Caron hidden in her ski bag.

Will encourages Elin to relax. They go to the hot tub, but they get into a fight. Elin walks by the icy plunge pool and someone pushes her in. On the third day, the hotel is evacuated. Elin and Will plan to leave on the fourth and final bus. After the third bus is loaded up, a man outside screams. Elin runs outside and finds a body floating at the bottom of the water, tied to a sandbag. At first, they think it is Laure. However, when they pull the body out they discover it is Adele. Elin takes charge of the scene and starts compiling evidence. An avalanche blocks the road. The police tell Elin that they are unable to reach Le Sommet, and they give her permission to investigate the situation. Lucas Caron and his sister Cecile Caron help her conduct the investigation. Elin finds a glass box with three of Adele’s fingers. A bracelet with numbers is wrapped around each finger.

Elin interviews everyone at the hotel and learns that Laure and Lucas were having an affair. Cecile gives Elin access to camera footage and they see Laure spying on Elin in the spa. Elin also sees a person who looks like Laure pushing her into the pool. Elin begins to suspect that Laure is responsible for the murders. When she tells Isaac her suspicions, he is angry.

On the fourth day, Laure texts Elin and asks if she will meet her in the penthouse so she can explain. Elin decides to go to the meeting. However, when Elin is waiting in the Suite Plaine Morte, the lift comes up and Laure’s body is inside. Elin texts Will to tell him what happened, and then the murderer arrives and attacks Elin. Will arrives and frightens the murderer away.

Elin inspects the new crime scene and finds another glass box with fingers and bracelets, and a hidden memory stick. Elin opens the memory stick and finds redacted patient files from a psychiatric clinic in Germany dating back to the 1920s. Elin realizes the patient numbers are the same as the ones on the bracelets.

A hotel employee named Sara finds Elin and tells her that Margot was abducted from their room in the night. Elin searches for Margot and uses the Apple Find My iPhone device to track Margot to the maintenance rooms near the spas. Will says he does not want Elin to go alone, so he comes with her. They find Margot tied up. Elin tries to help her, but Margot stands up and attacks Elin with a knife while admitting she is the killer. Will defends Elin and is stabbed in the stomach.

On the fifth day, Elin finds an envelope that Margot dropped. Inside are the unredacted medical files. One of them belongs to a woman named Bette Massen, who is the great-grandmother of Margot. There are also photographs of five women being tortured by men in masks, and then buried in an unmarked grave outside the sanatorium.

Elin finds a secret door in the archive room that leads down to a tunnel. She goes inside and finds Margot, murdered. Lucas Caron locks Elin in the tunnel. Isaac rescues Elin and she runs to find Lucas. His sister, Cecile, has attacked him. Cecile admits that she convinced Margot, who was emotionally disturbed, to help her carry out the murders in revenge for the fact that Daniel raped Cecile when she was a teenager and Lucas never did anything about it. After Lucas once again decided to cover up the sanatorium's dark past, Cecile decided to team up with Margot to destroy Lucas and the hotel.

The police arrive and Cecile is taken into custody. Five weeks later, Elin and Will leave Switzerland and say goodbye to Isaac. An anonymous man informs the reader that he has been stocking Elin for months. A newspaper article informs us that over 30 bodies were found buried outside the sanatorium.