Lt. Eve Dallas and her assistant, Delia Peabody, are called in by Officer Troy Trueheart after he has shot and killed a suspect named Louie Cogburn.

On examining the scene, Eve finds the attacker's computer bears a message on its screen: "Absolute Purity Achieved."

Some time later, the EDD investigator working on Cogburn's computer turns violently enraged, injures Ian McNab, and then dies of the same intense brain swelling while holding Captain Feeney hostage.

The next day, another body is found, that of Chadwick Fitzhugh, an unconvicted pedophile, with the same swelling and evidence of the same uncontrollable violence, and with the same message on his computer terminal.

Following this, a text message is sent to reporter Nadine Furst from a group calling themselves the "Purity Seekers," claiming responsibility for the deaths of Cogburn and Fitzhugh, and promising more "executions" of criminals that the law has not been able to touch, by means of a unique computer virus.