Homicide detectives Ryan Walker, Nora Houghton and Lt. Ben Carroll try to close a string of strangulation murders. Carroll and his divorced wife, Bonnie, decide to stop sleeping together. She confides in him that she's seeing another man. Walker jeopardizes the investigation when he begins seeing a conniving television reporter.

Walker is assigned to head up a task force on the strangler. Nora's brother Jack steals information and leaks it to the reporter Holly Bennett after she seduces him. Bennett places herself in harm's way when she agrees to meet the strangler. When Nora and Walt are ambushed, Nora shoots the gunman in Chinatown shoot-out, but also inadvertantly shoots and wounds Walt. This prompts Nora to consider leaving the force.

When the verdict of a man convicted seven years ago of murdering his wife is reversed, Ben risks losing his badge to gather evidence to convict him of murdering his son (whose body was never found) as well. In another case, Nora and Walt struggle with a witness who agrees to testify against the man who murdered her boyfriend but reverses her decision when the murderer escapes from jail and threatens her.

Ben and Ryan team up with Theresa Baldoni, a tough New York cop, who comes to California to catch the man who murdered another cop; only she's not interested in justice, she wants revenge. Carroll becomes romantically involved with Baldoni. Nora and Walt protect a young girl orphaned after a hit man kills her mother.

As Walt prepares to retire, he looks into a 13-year-old murder case and learns he arrested the wrong man. The department throws Walt a going-away party. Ben teams Ryan with Nora. Ryan hopes to entice Nora romantically, as they investigate a suspicious suicide. Ryan and Nora uncover that the suicide was actually a murder with a dentist for the prime suspect.

The unit investigates when a policeman is murdered in bed and quickly finds a suspect, his wife, Andie Harper! Nora worries that the murder may have been too easy. Ben concurs. The squad reopens the investigation. Ben and Ryan look at each other suspiciously, because both once had an affair with Andie Harper. They both investigate and find out how sexually voracious Andie Harper is.

Ben, Ryan and Nora investigate the apparent hate-crime murder of a Jewish boy, but discover it's more complicated and that it might have been the work of some well-off teenagers. When an unidentified beggar is murdered, Walt becomes obsessed with finding out his identity.