Yasuko is a single mom raising her teenage daughter, Misato. Previously, she divorced a man she had first thought would be a great step-parent and a loving husband to her; however, he had turned ugly. She moved away and took up a new job in a sandwich shop near the Kiyosu Bridge along the Sumida River by Tokyo. She and her daughter live in an apartment complex across the hall from a high school math teacher, Ishigami.

Ishigami has never married. A genius, Ishigami thinks very methodically. He follows the same routine day in and day out and observes the homeless people along the Sumida as he walks to work each day. He makes a daily detour from his path to stop in at the Benten-tei for a sandwich just so he can see Ishigami. He never speaks to her other than a simple greeting, but he is in love with her just the same.

All is calm for Yasuko until her ex-husband walks into the sandwich shop. He has found her. He says he wants her back and presses the situation; however, he is really after money. Yasuko agrees to meet him after work and tells him this is the last money he will ever get. He later shows up at her apartment, having followed her home. Misato arrives and terrified, retreats to her bedroom. Yasuko's ex makes innuendos that he will do something to Misato as she is now older. Yasuko knows that she will never be rid of him. As her back is turned and while her ex is bending over to tie his shoes, Misato rushes from her room and strikes him on the back of the head. He falls then staggers to his feet in anger and lunges at Yasuko, knocking her into the wall. He grabs Misato and knocks her on the floor then straddles her, trying to kill her. Yasuko panics. She must save her daughter. She pulls the electric cord of a heating table and wraps it around his neck and pulls it tight. Her daughter keeps his hands away from the cord and in a few moments the struggle is over for he is dead.

Yasuko knows she must turn herself in to the police. But her neighbor, Ishigami knocks on her door. She hides the body, but Ishigami knows she has killed the man. She wonders how he knows. He convinces her to let him help. He will take care of everything and give her explicit instructions as to what to tell the police if they ever come ask her questions about her ex-husband. Yasuko thinks of her daughter and reluctantly agrees to Ishigami's plan. She soon learns that she may now be indebted to him forever and not allowed her own life.

Detective Kusanagi is playing chess with his friend, Professor Yukawa, at the university when he gets the call about a body found by the river. Yukawa, a physicist, has helped Kusanagi in the past with previous cases. When Kusanagi follows the trail to Yasuko's apartment, he speaks with Ishigami and learns that he attended the same university as Kusanagi and Yukawa. He learns that Yukawa and Ishigami were friends in college and that the man is also a genius. So starts the battle of wit between Yukawa and Ishigami as they reunite. Yukawa does his own investigation while leaving Kusanagi to figure everything out with little help from his friend.

Keigo Higashino, Japan's biggest bestselling novelist, has written a well-developed mystery. THE DEVOTION OF SUSPECT X is thrilling. Each clue is presented along with the reasoning in a progressive and methodical fashion until the dynamite ending. I thoroughly enjoyed reading this sharp and stimulating mystery.