Fans of John Lescroart know one thing: The master storyteller can step away from his hugely successful Dismas Hardy series and his work still doesnt disappoint. His newest stand-alone novel, Fatal, is no exception.

The story centers on two friends, Kate and Beth, San Francisco women in their 30s whose lives are transformed by a single impulse: Kates sudden desire to seduce Peter, a new friend of her husbands.

Kate has been happily married for years. Her husband, Ron, a wealthy lawyer, is an excellent lover, father and friend. Kate has never cheated on him, and theres no reason she should give in to a foolish moment of temptation now, but she cant seem to stop herself.

Frightened by the obsession, she confesses her desires to Beth, her one friend who is refreshingly outside her husbands lawyer clique in Pacific Heights.

Beth is smart, tough and kindhearted, a homicide cop who has seen too many murders at the tragic end of infidelity. (Conveniently, she is investigating just such a case when Kate reveals her desire.) Alarmed by the confession, Beth warns her to protect her marriage.

Naturally, Kate doesnt listen. She seduces Peter lavishly and quickly regrets it. When Peter wants to continue their affair, she shuts it down. But too late the damage is done, and Peters life begins to unravel horribly. When, six months later, his body washes up near the Cliff House, the case goes to Beth.

Lescroart has always found a fine balance between his two favorite genres: police procedural and legal drama. Fatal succeeds with a new pairing: It is a psychological thriller in bed with a homicide investigation. Given that the author is known for his well-developed characters, it seems natural that he would move into this territory. The characters of Fatal may be lawyers, but their inner lives matter here, not their courtroom dramas.

The books horror comes from its delicate grasp of emotional realities that are inexplicable but painfully real. The mystery of Kates destructive impulse is never fully revealed but beats throughout as the storys dark heart. In Peters psychological collapse, we see the twisted repercussions of infidelity: He realizes that by cheating on his wife, he has made a fool of her and will never respect her again. His disappointment spreads into a dangerous nihilism and rage.

Ron, Kates perfect husband, is too smart to be fooled, but his kind and decent confrontation with Kate runs as cold as ice. Even Beth becomes entangled in an unnerving side story, trying to rescue a grieving woman who struggles with a deadly eating disorder, which only seems to underscore the fact that heroes are sometimes just regular people floating from crisis to crisis in order to avoid having to fix their own broken lives.

The investigation happens slowly, allowing us time to savor the way it crushes everything in its path. There is a horrible feeling of inevitability as these characters wake up to the fact that their lives are unfulfilling, that they dont respect their families, that their spouses are controlling, that the whole system of matrimony and family has innumerable small leashes that can contain even the wildest beasts.

Fatal reminds us that guilt, duty, responsibility, compassion the components of a civilized life stand in brutal opposition to the pure and exalted sexual desire Peter and Kate find in a hotel room. Lescroart wants to pick a side in this battle, but hes too good a writer to moralize. The result is a dark, disturbing, satisfying read.

Zo Ferraris is the author of the novels Finding Nouf, City of Veils and Kingdom of Strangers. Email: books@sfchronicle.com

Fatal

By John Lescroart

(Atria; 352 pages; $26.99)

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'Fatal,' by John Lescroart - San Francisco Chronicle