The most dangerous day of the year – Valentine’s Day – has the team on high alert, and Officer Nolan invites everyone over for a party now that he is single.

Officer John Nolan's mom makes an unannounced visit that complicates his life, and Sergeant Grey considers retiring.

Officer Jackson West’s relationship with his new training officer, Stanton, has escalated and he begins to work with Sergeant Grey to find a solution.

Sergeant Grey supports Officer Nolan’s desire to become a training officer and offers to ride with him for the day to get him ready. Meanwhile, Tim and Lucy must manage expectations in their respective relationships as they decide on their plans for Mother’s Day and a romantic vacation.

Officer John Nolan is once again face-to-face with serial killer Rosalind, and she wants him to know she's not done with him yet. Meanwhile, officers Bradford and Chen work undercover with Detective Lopez and the Las Vegas Police Department to arrest leaders of a major criminal enterprise.

Officer John Nolan and the team investigate the unexpected death of a suspect in police custody. Their search also uncovers a very delicate loose end that could jeopardize Officer Celina Juarez's career. Meanwhile, Sergeant Grey and his wife, Luna, go to New York to visit their daughter, Dominique, only to realize she never made it home from the night before.

After one of their own is shot, the team suffers a series of close calls and realizes their division may be a target for a group of masked assailants.

The team comes together to celebrate John and Bailey’s wedding; meanwhile, Celina discovers a discrepancy in her case, leading to a new discovery. Elsewhere, Lucy and Tim’s relationship is put to the test.

Nolan and Bailey’s honeymoon is more of a nightmare than a dream when it turns into an active crime scene. Meanwhile, Tim and Celina partner up and must uncover the identity of John Doe.

When a toddler is found at the scene of a crime, John and Bailey must decide whether to let the child go to a shelter for the night or care for her themselves. Meanwhile, when someone from his past returns, Tim disappears and leaves Lucy in the dark.

After a mafia-related mass casualty, the team is tasked to keep the peace at the hospital. Lucy and Celina work together to investigate the suspects behind the attack. Meanwhile, Tim and Aaron embark on a metro ops mission.

The team searches for a serial killer as Nyla struggles with the aftermath following the attack. Lucy’s relationship with Seth takes a turn, while Bailey battles her fear of Jason Wyler.

It’s Valentine’s Day, and Lieutenant Grey gives Tim and Lucy an unromantic assignment, while John and Celina track down a missing girl. Later, the team sharpens up for a charity gala where multiple relationships come to a head.

John, Lucy and Angela investigate the suspicious stabbing of three teenage girls. Back at the station, the team experiences technical difficulties while managing a group of prisoners and a wild animal.