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An alliance between a detective and a criminal create deadly consequences.

The investigation of a police officer's murder shows he is allied with a notorious criminal.

After a second young mother is found sexually assaulted and murdered in her own home, the squad begins trying to link the crimes together. Unfortunately, everything seems to lead to one of their own. Castillo comes close to compromising the investigation because she has trouble dealing with the sexual assaults, and Pogue, despite his initial misgivings, enlists the help of the police techie to hunt down a viable suspect.

A shooting at a dance club leads to a family being murdered.

After the skeleton of a young man is found in an old apartment house, Pogue initially suspects that the man may have been killed in the Rochdale College riots, but a composite recreation leads detectives to a twenty-five year old missing person, Liam Morrison, who has not been seen since late August of 1974. While investigating the murder, they learn that Liam was an informant for Pat Berlinger, a legend in the police department. Further investigations lead them to the discovery that Barrow was partnered with Berlinger on the night that Morrison disappeared.

Bodies of women are found in a warehouse.

Mistaken identity is the probable cause of a drive-by shooting.

When teenager Andy Kralic is found dead on the subway tracks, officers initially assume that he was murdered before surveillance videos show that Kralic walked in front of the tracks himself. Barrow is put in a hard place by Dr. Gordon Stiles, from the coroner's office, who is looking to do an inquest into recent deaths and wants answers about Kralic's suicide. Pogue and Oosterhuis question Andy's friends, Patrick Lee and Linda Ferrero, about Andy's life and school and learn that he had recently gotten in trouble at school for an article the three of them had never intended to put in the paper. Castillo, Pogue and Oosterhuis all disagree on Andy's death, with Castillo convinced someone else is to blame, Oosterhuis convinced that something drove him to it, and Pogue convinced that sometimes there is no reason.

The Russian mafia is suspected of a celebrity's shooting.

The officers discover that mysterious deaths at a hospital may be somehow linked with unsolved deaths. Castillo goes undercover at the hospital to investigate their prime suspect, a beautiful and compassionate doctor, but the more Castillo gets to know her, the less she wants to find Christina guilty of murder. In a final attempt to bring the killer out of the shadows, Tarver goes undercover as a patient in the hospital, with Pogue and Oosterhuis watching on video.

A racial killing is suspected when a black civilian is shot.

Four abortionists are killed and the killer's next target is posted on a website.