This novel focuses on three main characters - Bill Hodges, a retired police detective, a black teenage boy Jerome, and a neurotic woman Holly. These three characters form an unlikely team to stop the antagonist known simply as Mr. Mercedes from committing his second massacre after the infamous City Center Massacre.

The first few chapters of the novel focused on the bleak economy, when thousands of job applicants gathered for a chance to get interviewed and land a job. Amidst the long queue, there is also a mother with her baby, who she carried around because she was not able to get a babysitter for the day. As the mother laid inside a sleeping bag with her baby cuddled together for warmth, a Mercedes crashed through the crowd, killing 8 - including the mom, the baby and a man trying to protect them from the impact.

Mr. Bill Hodges was on the case, but not for long. 

After his retirement, Mr. Bill Hodges spends most of his day in front of the TV, contemplating using his gun on himself. On one of those days, he received a letter from someone who claimed to be Mr. Mercedes.

The letter detailed how he felt when he drove through the crowd and what he did to prevent his DNA from getting all over the car. He stole the car from Olivia Trelawney, a rich woman staying in a prestigious area called Sugar Heights, and made sure that the retired cop knew that he is being watched. The words were not very clear, but the intention was there - the villain wants to push Bill Hodges, the lead detective for his case, to his suicide. 

Bill Hodges did not take the bait.

He found a renewed interest in the case, and wanted to catch the villain despite Mr. Mercedes claiming that he is not interested to do the same massacre again. 

Bill remembers the interviews he had with the owner of the car of interest - the Mercedes that Olivia Trelawney forgot her keys in. She seemed appalled at the idea that she left her keys in the ignition - leading to the lives killed.

The more Olivia denied the possibilities, the harder Bill Hodges and his cop buddy Pete Huntley pressed on. Eventually, the two cops started to put less faith in Olivia. Eventually, she committed suicide.

Olivia's death pushed Bill Hodges to take extra care of the case once more. He started by questioning several individuals, one of whom included Janelle Patterson, known better as "Janey" in the book.

Janey showed Bill a letter that she believed was from Mr. Mercedes, having taken a liking to this retired cop. Soon, they were caught in a romantic relationship together, although Bill Hodges did not reveal this to anyone.

Meanwhile, Mr. Mercedes, known as Brady Hartsfield to the readers, have been paying attention to what was going on. He worked two jobs - one as an IT specialist that visits customers' homes to fix their PC issues, and the other as an ice-cream man right in the neighbourhood where Bill Hodges and Jeremy stayed.

On one of his rounds, Brady noticed Bill's car parked outside Janey's apartment, which was a turning point to make him become more impatient to take action - any form of action to show that he was a real threat.

Bill Hodges looped Jerome in on Debbie's Blue Umbrella, the website that Brady Hartsfield invited both Olivia and Bill to so that they can talk in private.

Soon, Bill was sending taunting messages to Brady on the site, cleverly provoking him in a way that he knew Brady would not be able to handle. He denied the possibilities of Brady being Mr. Mercedes, using another convicted serial killer that was recently acquitted.

Brady was riled up, and had one of his bad headaches that happen whenever he's stressed or thinking about his dead younger brother - the first murder he committed with his mother. Here, it was revealed that Brady was involved in some sort of incest with his mother.

He prepared a burger with poisoned meat that was meant to be given to Jerome's pet dog. The intention was to show Bill Hodges that he was the real deal, and that Bill should give him the credits and attention that he sought.

The plan failed when Brady's mother had one of her good days out of her alcohol fever. She found the hamburger packed in the fridge and ate it. Brady ignored her pleas for help when he found her, sad that she had to go that way but unwilling to help due to the fact that he would be caught if he did. 

His mother's death drove Brady almost mad. He told the cop only one thing - that he would kill him.

Brady used a device that he made to break into other people's cars - a device that steals a car remote's signals and mimics it to unlock the car without a key. He planted a bomb in Hodges' car during Olivia's funeral, which was attended by Olivia's relatives, Janey and Bill Hodges himself.

Bill would have been killed at that point, but Brady mistook Janey for the cop when she wore the hat that she gave him and drove away in his car.

Janey was killed in the explosion.

The explosion brought Holly - Janey's cousin - closer to Bill Hodges and her own sanity. Together with Jerome and Bill, the three started to uncover more and more of Mr. Mercedes' identity.

They checked past car burglary logs and several other crime records. When they managed to open Olivia's computer, the truth started to unravel. Things moved fast once the trio connected Olivia's computer to the tech store that Brady worked at thanks to a security guard in Sugar Heights.

Jerome immediately knew who Mr. Mercedes was when he saw Brady's familiar face listed on the tech company's website - the ice-cream man that his sister and himself often visited for their snack. 

They managed to find Brady's address, from which they also found his mother's dead body and her record of the passcodes for Brady's basement area. Upon unlocking Brady's computers and e-mails, they found what they feared - Brady was going to kill even more people in a concert that was going to happen that very evening. There would be more than 4000 people in that stadium, including Jerome's sister and mother.

Brady had disguised himself well, entering through the handicapped area in a wheelchair, carrying his younger brother's picture in his lap to hide the blinking lights of the bomb he planned to detonate. While Bill had alerted his friend working in the security area of the concert, the retired cop also did not suspect Brady of being the culprit.

Bill, Jerome and Holly later arrived at the concert hall when the music was just starting. While Bill succumbed to signs of a heart attack, Holly and Jerome went ahead to find and subdue Brady. Holly, who had gained much more confidence throughout the ordeal with Bill and Jerome, spotted Brady and knocked him out with the handmade weapon that Bill always carried around - the Happy Slapper.

Brady fell unconscious immediately, and Holly pulled the batteries off the electronic detonation device. The concert and its 4000 attendees were saved.

In the epilogue, Jerome and Holly were awarded as heroes of the city. Holly moved out from her family home, away from her eccentric and overbearing mother. Bill was questioned upon recovery from his heart attack, but he was still a free man.

Meanwhile, in a hospital, Brady woke up from a coma and asked for his mother, saying that he had a headache.