Shutter Island, Dennis Lehane’s 2003 psychological thriller, investigates how a person’s mental state can shape their perspective of reality. The prologue opens with an excerpt from Dr. Lester Sheehan’s diary dated May 3rd, 1993. Sheehan is a retired psychiatrist from Ashecliffe Hospital on Shutter Island, but he remains haunted by the dangerous tragedy of four people: Edward “Teddy” Daniels, Rachel Solando, Dolores Chanal, and Andrew Laeddis. Now that Sheehan’s health is failing, he feels compelled to bring the story to light.

The events of Shutter Island take place over the course of four days in September 1954, and the novel is divided into four sections that reflect the events of each day. The story itself follows Teddy Daniels, a World War II veteran and current U.S. Marshal, who is sent to Shutter Island to investigate the disappearance of Rachel Solando, one of Ashecliffe’s patients. Teddy finds himself in the company of his new partner, Chuck Aule, a recent transfer from the Seattle bureau of the U.S. Marshals. Chuck is likable and charming, and they immediately establish a rapport.

Teddy briefs Chuck on their case as they make their way to the island. Teddy explains that Ashecliffe is a federal hospital reserved for the criminally insane, and all the patients are both mentally ill and dangerous. To make the situation more difficult, a tropical storm is rolling in. The storm will hit the island within hours of their arrival, and it will ultimately trap them there for the next four days.

Teddy and Chuck arrive at Ashecliffe Hospital and begin their investigation. They meet with Dr. John Cawley, the lead doctor, who explains the situation: sometime the night before, Rachel Solando escaped her room and remains missing. Teddy and Chuck interview the staff, but the only clue they uncover about Rachel’s whereabouts is a written code she leaves behind, which Teddy and Chuck call “The Law of Four.”

That night, Teddy has terrible dreams about his wife, Dolores Chanal, who died in an apartment fire two years earlier. Her death remains a deep-seated trauma Teddy refuses to address. While it initially seems like he and Dolores had a love story for the ages, over the course of the novel, Teddy’s dreams reveal that their marriage had become rocky before her death due to Teddy’s alcoholism and Dolores’ mental illness. Teddy’s dreams serve as a critical motif in Shutter Island, where dreams reveal truths the conscious mind refuses to accept. Luckily, Teddy’s dream helps him break Rachel’s first code, which reveals that there is yet another missing patient on the island—the mysterious Patient Sixty-Seven.

The next day, the marshals interview patients who were in Rachel’s group therapy session. Most of them are unhelpful except for Bridget Kearns, who scrawls a one-word message on Teddy’s notepad: “run.” Afterward, Chuck asks Teddy why he asked every patient about a man named Andrew Laeddis. Teddy confesses that Laeddis is the mentally-ill arsonist who started the fire that killed Dolores. Teddy is certain he is at Ashecliffe despite there being no record of him, so Chuck suggests they check the graveyard, to see whether or not he might have died.

On the way to the cemetery, Teddy and Chuck find another clue from Rachel. She has stacked rocks into piles, and Teddy writes the numerical code in his notebook. The weather worsens rapidly, and Teddy and Chuck have to take shelter in the mausoleum. While there, Teddy tells Chuck the full truth of his mission. An inmate named George Noyce told Teddy about inhumane and illegal experiments happening at Ashecliffe. Teddy has been sent by a U.S. senator to investigate Noyce’s claims and expose any wrongdoing. Chuck tells Teddy that he believes they have walked into a set-up. Teddy tries to reassure his partner and promises they will leave the island.

The hurricane leaves the marshals stranded on Shutter Island. They also learn that they are caught in another battle. Cawley tells them that the psychiatric field is at war. The old guard believes surgery and confinement are the best treatment for the mentally ill, while a new wave of thinking believes psychotropic drugs are the best means of treatment. Cawley worries about both; he is a psychotherapist and advocates for talk therapy. These tensions within the mental health field play out thematically throughout the novel, including in its startling conclusion.

Teddy and Chuck return from the graveyard to find that Rachel Solando has mysteriously reappeared. They go to interview her, and she mistakes Teddy for her husband, Jim. Teddy gets sucked into her delusion, but the illusion shatters when Rachel suddenly realizes Teddy is a stranger. She attacks him, which triggers one of Teddy’s migraines. Cawley offers him some pills to alleviate the pain, and Teddy takes them against his better judgement. He has terrible nightmares that night about Dolores, and the strain of sleeplessness and stress begins wearing on him.

The storm passes through, but it has caused major damage. Teddy and Chuck seize the opportunity to explore Ward C, where the most dangerous patients are held. Once there, Teddy and Chuck split up: Teddy searches for Laeddis while Chuck tries to find patient files. Teddy finds George Noyce instead, who warns Teddy that everything around him is an elaborate hoax put on by Cawley. Noyce also tells Teddy that Chuck works for Ashecliffe.

Teddy and Chuck decide to investigate the lighthouse, which Noyce says is the surgical ward. On the way, Teddy climbs down a cliff to record another of Rachel’s codes, but he returns to find Chuck missing. At this point night has fallen, so Teddy shelters in a cave. There he meets the real Rachel Solando, a former doctor at Ashecliffe who was forced to flee when she uncovered the hospital’s illegal experiments. She tells Teddy that he has been drugged and gives him the list of symptoms, many of which he is already experiencing. After speaking with Rachel, Teddy returns to Ashecliffe in search of Chuck. Instead he finds Cawley, who tells him he arrived on Shutter Island alone.

Teddy plays along with Cawley so he can escape in the middle of the night, fully intending to catch the morning ferry to the mainland. But Cawley and the warden figure out his plan, so Teddy makes his way to the lighthouse instead, determined to find the truth and rescue Chuck. The only thing he finds in the lighthouse is Cawley, who tells Teddy that he is actually Andrew Laeddis. He builds a convincing case and introduces Dr. Lester Sheehan, who has been pretending to be Chuck the whole time. Cawley explains that Dolores was mentally ill and killed Teddy’s three children, and Teddy—who is actually Laeddis—shot her in the stomach. Unable to cope with what he had done, Teddy became delusional and was sent to Ashecliffe. But Teddy is dangerous, and unless he realizes the truth, he will receive a lobotomy. It is not until he dreams that night that Teddy remembers the whole story, and when he wakes up, he admits that he is Andrew Laeddis.

At the end of Shutter Island, readers question whether they should believe Teddy’s story—that he is a marshal who stumbled on a government cover-up—or Cawley’s narrative, in which Teddy is a delusional murderer. The concluding chapter offers no clarity. Initially, it seems like Teddy has accepted he is Andrew Laeddis, but then he starts talking to Chuck—who might be Dr. Sheehan or a hallucination, it is unclear—about getting off of Shutter Island. In the final scene, Cawley and a group of orderlies advance on Teddy with a straightjacket, though Teddy still believes he can escape.