After meeting with federal agents in a dark edge of Chinatown, Jamie witnesses a crime in progress and his suspect is killed while in pursuit. Now, Danny must solve the case in order for Jamie to avoid an Internal Affairs investigation.

A high-end escort's body is found in the river; Frank vows to get justice in a 25-year-old case.

Frank's frustration rises when the media sensationalizes the murder of a tourist shot and killed outside a trendy Lower East Side restaurant.

Frank is put on the spot when a supporter of the mayor is found dead and the mayor wants Frank to explain it as a random act of violence to the public. Meanwhile, Jamie goes on his first undercover assignment at a bar where he meets and bonds with a suspicious patron.

Frank must race to stop a biological weapon attack on New York City, while keeping it a secret from his family during their most sentimental holiday, Mother’s Day. Meanwhile, a long-standing conflict between Danny and Jamie comes to a head.

A woman recants a domestic-abuse claim, but Danny and Jackie continue to investigate the situation. Soon they uncover the woman's secret affair with a powerful politician who's friends with Frank. Meanwhile, Linda returns to the workforce, causing chaos at home.

When two motorists driving gas-guzzling cars die at the hands of a sniper, Danny and Kate are put on the case of what the city believes is the work of a radical environmental serial killer. Meanwhile, a bicycle accident sends Danny’s son Sean to the hospital with a life-threatening injury.