Equation for Evil examines the anatomy of a horrific crime: a lone gunman, Duane Boggs, has opened fire on a busload of Asian-American children in rural California, then turned the gun on himself after the attack. In response to public outcry, Gabriel Chin, special agent for the California Department of Justice, and forensic psychiatrist Leander Heartwood are ordered to conduct a "psychiatric autopsy" of Boggs to determine his motive and state of mind at the time of the attack.

Their investigation leads them through Boggs's sordid history, his involvement with white supremacist groups, and his teenage years on the streets.

But the trail takes an unexpected turn when they begin to suspect that Boggs was driven by something more than his inner demons. Following hunches, Chin and Heartwood are led to Boggs's half brother and then to Mace Weathers, outwardly a personable college student but in reality a man compelled by inner forces that may be more malevolent than the killer's.

With the help of rural sheriff Phil Youngblood, evidence specialist Jean Sheldon, and Joyce DeLuca, a teacher and survivor of the massacre who must come to terms with the horror, Chin and Heartwood set out to unravel what really lay behind the crime. Along the way Chin encounters shocking realizations about his own violence and prejudices, while Heartwood confronts a disturbing truth: There are evils in the human spirit that psychiatry and medicine can neither explain nor cure.