Season 4

Firefighters find a mass grave in the cellar of a home with nine bodies, one of which is directly connected to a cold case. Christine has trouble fitting in when she takes over for Bernice, who didn't leave her any instructions. Mickey comes under suspicion when Bailey's wife is murdered because he pointed the finger at her.

The squad re-investigates the murder of a well-known philanthropist at the request of the son who is believed to have murdered him. Nicco helps Mickey when she's suspected of murdering Bailey Gallanson's wife.

A count of the corpses in storage at a police training facility leads to a murder investigation.

The squad helps Moscow police probe a Russian hockey star's death.

Nicco is in the hot seat when his friend Gus (another cop) is shot and killed, and a local drug dealer, Hoppy, is charged with the murder. Ali and Mickey investigate when a thief about to be released from jail confesses to a murder, believing he simply wants to remain in jail.

A tax-evasion case leads to an investigation of a prostitution ring and murder.

Mickey and Ali investigate a twelve-year old murder that was believed solved thanks to a tip from a junkie named Simon. Coscarella investigates when two dogs trained by the same man attack people on command. Christine cannot resist the lure of her past, and steals cocaine from the lock-up.

Courts rule evidence in a murder case inadmissible prior to a retrial.

When one of Reid Logan's five wives shows up at the station, she tells Frank and Nicco that she believes the death of Louise Logan, Reid's only legal wife, four months ago was foul play. While investigating, Nicco is intrigued by Libby, the most recent wife who took Louise's place as Reid's only legal wife. Mickey is beaten up by a suspect in the brutal deaths of the Williams' and their baby, and although Ali tries to help her, she soon crosses the line in her zeal to get Price and his partner.

Ali investigates the death of Philip Morris, a teen who was believed to have committed suicide by jumping off the Capilano suspension bridge six years earlier. Coscarella worries his son Leo may be gay after Frank sees him embracing another guy after returning from a rave.

Police corruption may have played a role in drug-gang activities.

Frank and Nicco are stripped of their badges over the drug probe.

Frank investigates the death of a forgotten war hero. Ali applies to become a Big Sister after questioning her value as a cop

Two robbery suspects take hostages in the squad room.

The squad tries to break a former officer preparing to leave the country. They're convinced he killed two people while his young daughter, dying of cancer, watched from the back of the car.

Thirteen-year-old Kevin Lane disappeared on June 6, 1979. Years later, his parents discovered that his former baseball coach Harry Walters was really pederast Martin Woodridge, his mother became convinced that Woodridge murdered her son and hid the body. Woodridge insists that he is all about consenting actions between adults and young adults, insisting that he has never forced anyone to do anything they didn't want to. After the Lanes offer a $100,000 reward for information, Woodridge comes forward and points the finger at himself. Although Ali would love to lock him up and throw away the key, she just can't do it.

A cold case turns warm when a mysterious man collapses near a hospital.

A self-help lecturer may have influenced a prison guard to murder a prisoner.

Cold Squad is convinced they have the necessary ammo to bring down killer Dean Bulmer, and Nicco goes so far as to promise the family of one of his unlucky victims that it's in the bag, but when a body found proves to belong to the man Bulmer is on trial for killing, the Squad realises that Bulmer has been fingered for the wrong murder, and it comes down to the moral question of whether to lie to keep a vicious killer in prison, or to tell the truth and have him set free.

When Ali's testimony results in the release of a killer (Dean Bulmer), Coscorella takes it too seriously and makes catching Bulmer his sole goal, much to the displeasure of homicide detective Len Harper. Nicco is furious with Ali and believes she should have lied to keep Bulmer in jail, and tries desperately to keep the father of one of the victims from taking justice into his own hands. Cold squad is in danger of being closed down, and the officers absorbed into the homicide division.