The CSIs investigated a possible serial killer when a body they found matched the M.O. of another murder. A shoe print and trace from the victim's fingernails led the CSIs to Charles Martin. When they arrived at his apartment they found that the scene had already been processed. Evidence proved that the victims found were killed in Martin's apartment, and evidence of a third woman also turned up.

Flack and the CSIs followed the evidence to a private investigator who had processed the apartment while looking for his client's wife, Christina Marino. Charles Martin returned to his apartment and was arrested by the police. He admitted to killing two women, but swore he did not kill Christina. The CSIs managed to track the Christina to a hotel where they found evidence of her murder. Her husband, who she had just left, admitted to murdering her after he had been interrogated.

Mac and his former partner became the target of an ex-convict they had sent to prison.