The following version of the book was used to create this study guide: Connelly, Michael. The Dark Hours. Hatchette Book Group, New York, NY, 10104, 2021. Kindle AZW file.

Renee Ballard is a detective with the LAPD. As 2020 comes to a close, the department is facing anti-police sentiments that have prompted serious cutbacks, creating a shortage of manpower and resources in the midst of a global pandemic. Many of the officers are not willing to do anything more than what is absolutely required. Ballard continues to put everything into her investigations, and she is willing to fight to retain the cases that are most important to her. On New Year's Eve, Ballard is parked under an overpass in anticipation of the annual “rain of lead” when celebrants fire weapons into the air without any consideration of where those bullets will fall. Ballard is paired up with Lisa Moore, one of the many officers looking to do only what is necessary. When they are summoned to the scene of a man who was shot in the head, Ballard shames Moore into helping with the investigation. Within hours, Ballard knows that the victim, Javier Raffa, was murdered. He was a former gang member who left that life behind. However, he had a lifelong debt to a wealthy group of men.

As Ballard begins to work the case, Moore goes away for the weekend in spite of being on call. Though Ballard is angry, she continues to follow leads on her own. She discovers the bullet that killed Raffa matches another unsolved homicide that was investigated by Harry Bosch. Bosch is a former detective who was pushed into retirement by job-related health issues. He is still active and quickly helps Ballard tie their two cases together. Ballard wants to solve this case, but she is facing the reality that she will be forced to hand the case over to the “dayside” detectives within a couple of days. As that deadline approaches, Ballard and Bosch follow the trail to a former confidential informant that leads them to a former police officer, Christopher Bonner. When Bonner tries to murder Ballard, she ignores proper protocol and takes the man's cell phone which leads directly to a group of dentists who are little more than loan sharks. By the time Ballard hands the case over, she has made one arrest which should clear the way to take down the entire operation.

While Ballard and Bosch are working this case, Ballard is also focused on a series of rapes. This situation is unusual because there are two rapists. Ballard soon realizes that the men are targeting specific women rather than choosing random victims. Following a series of clues, Ballard learns who the men are planning to target next. She has lost all respect for most of her coworkers and calls on Bosch for backup rather than going through proper channels. As she waits in the home of the rapists' next victim, she decides to take Bosch up on his offer to partner up in the private sector. She emails her resignation at that moment. When the two men enter the house, Ballard realizes her plan had some flaws, and she kills both men. She is questioned at the police station, but her union representative argues that Ballard was already a civilian when she killed the two men, meaning she was not required to follow police protocol and was within her rights to defend herself. Though police are ready to close this case, Ballard and Bosch continue to follow their instincts and learn that the victims' spouses had arranged for the rapes through the Dark Web. As the novel ends, they discuss what agency is most likely to follow this case through to punish those men.

In the Epilogue, Ballard is preparing to go surfing when the LAPD Police Chief arrives at her apartment complex. He feels it is time for change within the department. Having identified Ballard as one of the people who might make positive changes, he offers her job back. Ballard asks for a few days to think it over, then sets out for the beach with no one except her dog Pinto for company.