The following version of the book was used to create this study guide: Baldacci, David. The 6:20 Man. Hachette Book Group, New York, NY, 10104, 2022. Kindle AZW file.

Travis Devine had imagined he would spend his career in the military, but that time is cut short when he learns officials have covered up a murder. Devine fights with the murderer, and the murderer dies from his injuries. Devine leaves the military under a cloud and takes the worst job he can imagine – a low-level job at Cowl and Comely, a financial company. Devine soon suspects there is something illegal happening on the 51st floor – the floor no one that he knows can access. Then, Devine receives an anonymous email from someone telling him about the death of a woman named Sara Ewes. Devine and Ewes had secretly dated for a while, but she broke it off. Devine is shocked to learn Ewes committed suicide, but he soon learns police have ruled her death a homicide. Devine wants to know what happened to her, but he is interrupted by a meeting with a retired military man named Emerson Campbell.

Campbell works in the Office of Special Projects in the Department of Homeland Security. He knows about the fight that caused Devine to leave the military. He says Devine will face charges related to the fight unless he works for Campbell. Campbell believes there are illegal activities at Cowl and Comely, and he wants Devine to dig into the happenings there to help prove it. Devine reluctantly agrees, and he soon turns to two of his brilliant roommates for help. Will Valentine is a Russian immigrant who works as a hacker who tests security systems. Jill Tapshaw is an MIT graduate who has built a successful dating site. The third roommate is Helen Speers. She claims to be a recent graduate studying for her bar exam, but Devine much later discovers she also works for Campbell and is helping watch over Devine.

Initially, Devine believes that Ewes was murdered because she found something illegal at Cowl and Comely. He continues with that belief when a woman named Jennifer Stamos is murdered. Police are initially looking at Devine as a suspect, but Campbell clears the way for him to continue his work. Devine continues to look to Valentine and Tapshaw for help with technological answers. Devine learns that Cowl is laundering enormous amounts of money for many people, including foreign governments. Soon after the discovery, he knows that the rash of murders is not related. Devine comes to realize Speers is not who she claims to be, and his instinct is not to trust her. He also comes to suspect Valentine is involved with the money-laundering taking place at Cowl's business. Meanwhile, Devine has a series of clues related to the rash of deaths, but he does not have definitive answers. One night, Devine wakes to find the house filled with gas. He assumes Valentine tampered with the line and ran away.

Devine never considers that Tapshaw might be involved until he is searching for her phone while she is in the hospital after the gas leak. Through some documents and a confrontation with Tapshaw, Devine learns that Tapshaw's twin brother named Dennis recently committed suicide. Ewes and Tapshaw were in a relationship, and Ewes was pregnant due to artificial insemination with Dennis's sperm. When Ewes felt Tapshaw was spiraling because of Dennis's death, Ewes broke off their relationship and terminated the pregnancy. Tapshaw killed her out of revenge and then killed several others she feared had information.

As the novel comes to an end, Cowl is arrested but dies in jail. Devine has disrupted the money-laundering business, but it is not shut down. Devine catches the early morning train one last time as he considers what the future might hold.