We open with San Francisco Detective Lindsay Boxer rushing into a burning house where she finds three people dead, but is able to rescue a fourth. Almost killed herself, minutes later she discovers that another child, just a baby, and its nannie are missing.

There's also a mysterious message at the scene. When more corpses turn up, Lindsay asks her three friends, Claire Washburn of the medical examiner's office, Assistant District Attorney Jill Bernhardt, and Chronicle reporter Cindy Thomas to help her find the murderer who vows to kill every three days. Even more terrifying, he has targeted one of the four friends.