Mr. Mercedes is the 62nd novel book published by Stephen King; it is his 51st novel, and the 44th under his own name.
The novel starts with a scene in which jobless people stand in line for a job fair, when a Mercedes rides into the crowd and kills eight people and injures many severely. Immediately after that, the protagonist is introduced, Bill Hodges, a former police detective retired for six months. He is divorced, lonely and fed up with his life, occasionally considering suicide. Suddenly he receives a letter signed by a "Mr. Mercedes" who claims to be the Mercedes killer. The incident had taken place at the end of Hodges' career and was still unresolved when he retired. Mr. Mercedes knows details of the murder and also mentions Olivia Trelawney, from whom he had stolen the Mercedes; she committed suicide soon after. Hodges is intrigued and starts to investigate the case, instead of turning the letter to his former police colleague, Pete Huntley.
A new perspective in the novel opens with the introduction of Brady Hartsfield, the Mercedes killer. It is revealed that this emotionally-disturbed man in his late twenties had lost his father at age eight, lives with his alcoholic mother and works in an electronics shop and as an ice cream seller. Riding in a van, this second job enables him to observe Hodges and Hodges' neighbors, among them seventeen-year-old Jerome Robinson, who does little chores for Hodges.
During his research about the wealthy Olivia Trelawney, Hodges meets her sister Janey, who hires him to investigate Olivia's suicide and the stealing of the Mercedes. They become a couple. Hodges finds out, with the help of bright and computer savvy Jerome, how Mr. Mercedes had stolen the car, and that he had driven her to suicide by making contact with her and working on her feeling of guilt. At the funeral of Janey's (and Olivia's) ill mother, Hodges meets Janey's unpleasant relatives, among them Janey's freakish cousin Holly. After the funeral, Mr. Mercedes blows up Hodges' car accidentally; Janey was in it and is killed. Hodges feels remorse, but becomes even more eager to solve the case without the help of the police. Holly joins Hodges and Jerome in the investigation.
Hartsfield accidentally kills his mother with a poisoned hamburger which he had prepared for Jerome's dog. With her rotting body in their house, he plans to kill himself by blowing himself up at a giant concert for young girls; the concert will be attended by Jerome's mother and little sister. Hodges, Jerome and Holly manage to uncover Brady's real identity and search his computer hard drives. As they suspect a different location to be Mr. Mercedes target, they come late to the concert, but not too late. While Hodges has a heart attack and is taken to the hospital, Jerome and Holly succeed in preventing Brady from detonating his explosives.
In the epilogue, Jerome and Holly are rewarded with the medal of the city; Hodges is lucky not to be charged for his irresponsible conduct. Brady, who had been beaten by Holly into a coma, wakes up.