In the wake of violent targeted attacks on 15 Division, Andy, Swarek and their fellow officers begin to piece together a past crime in order to find out what has happened to Oliver Shaw. While the squad is desperately trying to find Oliver, Dov and Wes face off about what's best for Chloe. And as the shooter's plan is finally revealed, it may be too late for some of the officers of 15.

Andy and Chloe are blindsided by an unseen shooter when they respond to a routine 911 call. When more shots are fired at a subsequent emergency call, the officers realize that 15 Division is being targeted by someone out for revenge. The clock is ticking as the squad tries to serve and protect, while running down suspects. Meanwhile, Oliver comes face to face with the shooter in a gut-wrenching, life-changing moment.

When a suspected pedophile is found severely beaten, Andy discovers that Marlo has been carrying out an all-out, unauthorized investigation into the man. Andy and Swarek must scramble to cover for Marlo's unprofessional conduct before she is exposed. Meanwhile, Traci oversees a major drug sweep, assisted by Chloe, Nick, Dov and Gail, but when Sam leaves his post to help Marlo, the operation takes an unexpected turn.

A routine bust of a marijuana grow-op develops into Traci's first homicide case, and Andy, Nick and Gail are forced to work overtime to investigate the unusual murder case. Meanwhile Swarek, Oliver, Chris and Dov are off for a weekend of male bonding and generator repair at Oliver's new cabin. Both Andy and Sam both have something to confess, but is it too little too late for both of them?

Chris' reunion with his rookie family takes a shocking turn when his son, Christian, is kidnapped while in Andy and Nick's care. As the squad spreads out to search for the boy, Swarek is confronted with Andy and Nick's budding relationship. As our officers discover clues to the identity of Christian's abductor, Chris is slapped with a stunning revelation that might tear his new family apart.

Everyone at 15 Division is looking forward to celebrating at Frank and Noelle's wedding. But when a casino-bound bus full of seniors citizens is robbed at gunpoint, all bets are off. Meanwhile, the wedding preparations go awry when Traci discovers the bride has disappeared, and Andy's day takes a 180-degree turn when she and Nick share a strangely intimate moment.

The grisly discovery of human bones in a ravine re-opens a 15-year-old missing persons case, and Swarek teams up with his old training officer, Charlie Walsh, in an effort to close the case. But an up-close-and-personal look at Charlie's life makes Swarek think twice about whether he wants to continue in his mentor's footsteps. And a weird night gets even stranger when Chloe and Dov can't agree with how to deal with a witch and her disgruntled client.

The investigation of a missing girl is linked to a possible copycat of one of the squad's most gut-wrenching cases. Detective Luke Callaghan is brought back to 15 Division to help capture the abductor. With the clock ticking, Gail is forced to confront her worst fears in order to help solve the case. Meanwhile, Traci realizes she must move past Jerry's death, and Andy and Sam work together again, coming face to face with their past and the real reason for their breakup.

A chaotic drug arrest by Andy and Marlo triggers an all-out crisis when it's discovered that the drugs are contaminated with deadly bacteria. Swarek leads the squad in the scramble to get the tainted drugs off the streets before an outbreak sweeps the city. Meanwhile, Andy and Marlo are plunged into an intimate decontamination process, forcing Marlo to expose a shocking secret while under quarantine. And Dov and Chloe see their budding romance threatened when Dov suspects Chloe is hiding secrets.

Andy is ready to get away for a much-needed long weekend when she and Gail, working a routine highway spot check, discover a teenage boy bound and gagged in the trunk of a car. The two find themselves plunged into a messy tale of young love gone wrong and must try to save the boy from making a tragic mistake. Meanwhile, Swarek, Oliver and Chris are hot on the trail of a gunman who shot a 16-year-old who may have gang connections.

Andy and Officer Marlo Cruz, Swarek's new girlfriend, butt heads on their first patrol together. Their different ways of handling the job are put to the test when they confront an emotionally disturbed woman who is losing control - and explode when one of them is forced to make a crucial life-saving decision to pull the trigger. Meanwhile, the rest of 15 Division is hunting down an armed burglar with a novel MO after a string of ATM robberies, and Dov is forced to work with new rookie Chloe Price after their indiscreet first encounter.

Andy must make peace with her past when she's forced to work a bank robbery case with the newly promoted Swarek on her first day back at 15 Division. The investigation leads them to a violent ex-convict determined to get his old life back at any cost. Meanwhile, Nick and Gail help a distraught woman who offers Nick some perspective on his life with Gail, and Frank drops a bombshell on Noelle.

A group of young and ambitious rookie cops continue to navigate the emotional trials and tribulations of their professional and personal lives. After three years on the job, these rookies are well on their way to becoming experienced officers, but at what cost? It's been six months since Andy McNally and Nick Collins left for their undercover operation. But while they were gone, everything seems to have changed at 15 Division.