Darkest Fear, written by Harlen Coben, details the search by Myron Bolitar for the missing bone marrow donor of his biological son, Jeremy Downing. Myron is drawn into the search for the mysterious donor when his first love, Emily Downing, arrives with the news that his son is dying and needs a bone marrow transplant. Myron is reluctant to become involved with Emily, as his past with her and her husband Greg had been life altering for him. Emily reveals that Myron is the biological father of Jeremy; that he was conceived the night before he wedding to Greg. Myron is still reluctant, and now also angry, but agrees to help Emily find the donor.

Myron enlists the help of his friends and locates the name of the missing donor. He and Greg Downing investigate the lead and find it to be a dead end. Esperanza Diaz, Myron's partner at MB SportsReps digs deeper and finds that the donor had changed his name recently. The true identity of the donor appears to be the missing son of an extremely private millionaire. Myron contacts the family and is refused any information; in fact, he is threatened should he ever attempt to contact them again. Undeterred and determined to help Jeremy, Myron continues to follow where the information leads.

A mysterious phone call in the middle of the night draws Myron to another path. The caller quotes a serial killer that had been interviewed in a series of articles by Stan Gibbs. Gibbs had been accused of plagiarism when the FBI discovered a decades old novel with passages that were duplicated in his article. Despite the testimony of his then mistress that she too had met the Sow the Seeds killer, Stan was disgraced and left public life. Myron tracks him down in an effort to find the bone marrow donor. In a seemingly bizarre twist, Gibbs has knowledge of the Lex family, presumably because he had planned to write an article about the family.

The FBI becomes involved after Myron's meeting with Stan, trying to track down leads to the killer. Agent Kimberly Green informs Myron that the Bureau believes that Stan Gibbs is the true killer. They seem to be basing this on the fact that his mistress was murdered when she planned to recant her testimony. Myron is not sure if the FBI is lying or telling the truth, so he continues his search along with his friends, determined only to find the marrow donor and save Jeremy.

Events rapidly occur, culminating in the kidnapping of Jeremy Downing. Myron is left with no choice but to find Dennis Lex, the missing younger brother of Susan Lex and the presumed bone marrow donor. Arranging a kidnapping of his own, Myron whisks Susan away from her security detail and she drives him to the hospital where her brother has lived for more than thirty years. Susan knew that her brother could not be listed in the bone marrow registry because she had shot him accidentally when they were children. At a seeming dead-end, Myron spots a statue in the courtyard of the hospital, the same statue that Stan Gibbs had posed in front of with his father, Edwin.

Myron connects the clues and returns to Stan Gibbs who admits that his father is the Sow the Seeds killer. In an effort to find Jeremy, Stan takes Myron to the location where his father is hiding. Shots are fired but Stan bravely tackles his father so that he can still be a bone marrow donor. At the police station Edwin confessed his crimes to his attorney and makes a deal to reveal the location of Jeremy. While the main parties are searching for the boy, Edwin kills himself, seemingly ending all hope of the boy getting a transplant.

Events continue to escalate as Myron realizes that the facts do not add up. With a bit of help from the FBI Myron discovers that it is Stan Gibbs who had taken the identity of Dennis Lex, and registered to be a bone marrow donor. Myron makes a deal with Stan who donates his marrow secretly, and saves the boys life. Once Jeremy is safe, Myron returns to Stan with the FBI, able to prove the plagiarism charges that Stan had tried to vindicate himself from. Stan had arranged the entire affair so that he could turn his father in without looking like an ambitious, uncaring man. Along the way he had presumably killed his mistress when she discovered the book Stan's father had written in his apartment.

Myron moves on with life, beginning to pull things back together in professional life as well. Planning to reveal the fact that he is Jeremy's biological father, Myron is stunned when the boy tells him that he already knows. Reacting with the intelligence and feeling of a much older person, Jeremy tells Myron that he might be his father, but Greg is his dad. The boy would, however, like Myron to be a part of his life.