Blood Work by Michael Connelly is a mystery that has many twists and turns. In epic Connelly style, Terry McCaleb, a medically retired FBI agent who specializes in the profiles of some of the worst criminals is asked by a woman to help find her sister's killer. While at first McCaleb is reluctant, and tries to steer her towards others in the field who might help, Graciela Rivers is persistent. Her reason is clear as soon as she confesses that the heart transplant he received belonged to her murdered sister. Feeling that he at least owes the family that much, he agrees to at least talk to the detectives on the case, but he councils her not to get her hopes up. Thus begins a journey that will take Terry McCaleb from law abiding agent to suspect. In the many turns this case will take, Terry will also discover that the myriad of evil he has faced throughout his career has not permanently damaged him as he once thought, and when everything is over, Terry will begin to believe in angels again.

Terry begins with the detectives assigned to the case, but the lead detective Arrango is less than cooperative, seemingly more concerned with his reputation than he is with his solve rate. When he digs deeper, the case takes several unexpected turns, starting when he discovers this is no isolated robbery in a market. In fact, the same weapon had been used in an ATM robbery shortly before. The lead detective on that case is an old coworker of Terry's and is much more forthcoming with information and files. He digs deeper still and the twists and turns continue. Not only does he find a third victim linked to the same killer, but he also discovers that the killer is taking trophies in each case. Every time Terry gets close, another turn takes him further down the rabbit hole of the mystery involving the murder of Gloria Torres.

Just when it appears as though he is making headway, several incriminating items are planted on his boat, and an anonymous tip leads the investigating agents right to him. It looks as though Terry is headed for arrest as their prime suspect; but as is typical of his organization ability to see a case and work the details, Terry manages to find the inconsistencies and avoid prosecution himself. In a suspense filled ending, Terry finds the killer after he discovers that all the murders were committed to secure him and his rare blood type a heart transplant. The killer, an old nemesis of Terry's dubbed the Code Killer for the cryptic numerical codes he leaves behind at each crime scene, decided he missed the interaction with McCaleb when his heart gave out. Gloria Torres matches his blood type and is the killer's Valentine to Terry. The last straw is when the killer then kidnaps both Gloria's sister Graciela who has managed to warm Terry's damaged and discouraged heart, and her nephew Raymond. Terry will break the law and all the rules to save them and his one true chance at a normal life again.