The following version of the novel was used to create this study guide: Coben, Harlan, Don’t Let Go. Dutton, September 26, 2017.

In the novel Don’t Let Go by Harlan Coben, Detective Napoleon “Nap” Dumas learns the truth about the deaths of his brother, Leo, and Leo’s girlfriend, Diana, but also comes to realize that the truth can be painful. 15 years after the deaths of Leo and Diana, Nap delves into the connection between them and two other classmates recently murdered. An old military base located in their town of Westbridge, as well as a viral video, seem to have relevance to the deaths but Nap is unable to put all the pieces together until he talks to Maura, who has been missing since the night of Leo and Diana’s deaths, and Beth, who has more recently gone missing. Nap is devastated when he puts together all of the clues to identify his brother’s real killer.

In a Pennsylvania bar, Maura, using the name Daisy, picked out Dale Miller in the crowd. Because he was getting divorced and in a custody battle, Maura’s friend and police officer Rex had been hired to stop Miller and give him a DUI. It was Maura’s job to be sure Miller was drunk enough to be charged and then ask him to drive her home. Instead of charging Miller, however, Miller shot Rex in the head, killing him.

Nap is contacted by police from Pennsylvania concerning Rex’s death because he had put Maura’s fingerprints in the police system as a person of interest. He wondered immediately if Rex’s murder was connected to the death of Leo and Diana. Ellie, Nap’s best friend, pointed out to him that Rex, Maura, and Leo were all members of the Conspiracy Club, a secret organization, in high school. Hank Stroud and Beth Fletcher were also members of this club.

Nap looks for Hank, who still lives in Westbridge but suffers from mental illness, but is unable to find him. He asks Augie Styles, his friend and mentor in the police department, to have his officers look for Hank. Augie lets Nap know that Hank had come to the police department because he was being threatened after a viral video was posted accusing Hank of exposing himself to a high school student. A few days later, Hank’s body is found hanging in a tree. It is made to look as if he had been killed because of the viral video.

David Rainiv, a friend of Hank, gives Nap a video that Hank and the Conspiracy Club made while still in high school. It shows a stealth military helicopter landing at a site formerly used as a Nike missile base during the Cold War years. A man dressed in a prisoner’s outfit appears to get out of the helicopter. Nap learns the Hank had given the tape to David for safekeeping and told him to give it to someone he trusted in the case he was ever murdered.

Nap investigates the history of the base and learns that while he and the others were in high school, the base was supposedly used by the United States Department of Agriculture. He gets in contact with Andy Reeves, the man in charge of the base at the time. Reeves pretends the site really was used by the USDA until Nap shows him the video. Reeves threatens to kill Nap and everyone he loves if the video is ever released publicly. Nap talks to Reeves a second time and threatens to release the tape if he is not told the real purpose of the base. Reeves relents and tells Nap that the base was used as a black site to torture and interrogate American citizens who were terrorists. Nap tries to get more information but Reeves overpowers him and knocks him out. When Nap regains consciousness, he is in a deserted warehouse. Reeves begins waterboarding him, trying to get information about Maura. Maura appears and shoots Reeves.

Maura tells Nap she believes it might have been her fault that Diana and Leo were killed. She was drinking alone at the base the night of their deaths and was overcome by the urge to disobey the warning signs and run up to the fence at the base. As she approached the fence, the floodlights came on and she heard gunfire. She heard Diana screaming for Leo.

Nap put the final pieces of the mystery together when he talked to Beth, also a member of the Conspiracy Club, and then Augie. Beth had taken a leave of absence and was contemplating suicide as a result of the guilt she felt for her role in Diana’s death. Leo had learned that Diana was going to break up with him and planned a prank to punish her. Beth feels responsible for Diana’s death because she believes the prank, which ended in Diana running toward the fence at the base, was the cause of the teen’s death.

Nap realizes that Augie played a role in the tragedy and finally gets answers from the man who was his mentor. Augie responded to a call of a disturbance at the base and learned from Reeves that a prisoner had escaped. Because they could not let what they were doing at the base become public, guards turned on the spotlights so they could locate and shoot the terror suspect. A guard was spooked when Diana ran toward the fence and shot her as well. When Augie interrogated Leo and discovered he had played a prank on Diana that resulted in Diana’s death, Augie shot Leo. It was only 15 years later, after the viral video of Hank was released that Augie realized that Hank, Rex, and Beth were involved in the prank as well. Augie took matters into his own hands to get justice for his daughter. In order to cover up what was happening at the military base as well as his own crime of murder, Augie helped Reeves pull the bullets from Leo and Diana’s bodies and put them on the railroad tracks to make it appear they had been hit by a train.