Eddie Argo (Stellan Skarsgard) and his new partner Helen Westcott (Melissa George) are investigating a killer that has a special M.O. in which the victim is forced to kill someone close to them or die themselves. The experiment is set to prove people are inherently selfish.
After a few victims are discovered, Argo figures out that Jean Lerner (Selma Blair) is the killer. She had been kidnapping the people who broke into her house, raped her, and then gave her the choice of killing her mother or dying herself. She wanted to prove to herself that she had no choice in what she did.
Her final target is Daniel, a informant of Argo. She then targets Argo as her true victim. Argo was Daniel's lover, and helped destroy the case against him and the others. Jean tortures Argo to make him flick the switch on Daniel but he refuses to. Argo tells Jean to kill him, and she does, slitting his throat, saying "Thank you." That seems to imply that in Argo sacrificing himself, Jean realized that there is altruism in the world and it is not all bad.
Jean calls the cops on herself and is arrested. Daniel covers up Argo's body and sobs at his death. Wesstcott leaves the scene, distraught the killings went that far.