Danny and Jackie are assigned to protect a key witness in a Russian mob trial, but when an attempt is made on her life, they realize there is a leak on the task force. Meanwhile, Erin is offered a job as deputy mayor, which would have her working directly with her father.

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.