Andy and Nick continue to work undercover infiltrating a meth pipeline. However their covert mission goes sideways when first Nick and then Andy go missing. Meanwhile, a random arrest at 15 Division puts Sam Swarek and the other officers in the middle of the drug investigation-turned-rescue operation. Finally, Andy must figure out how to cope with the transformation of her own life as well as those of her friends at the station and continue to do the job she loves.
On her first day back, Andy is forced to work a bank robbery case with newly promoted Swarek; a distraught woman gives Nick perspective on his life with Gail; a bombshell is dropped on Noelle.
When Andy is forced to ride with Swarek's new girlfriend, Officer Marlo Cruz, they butt heads over different approaches to the job. But when they come face-to-face with an out-of-control EDP [Emotionally Disturbed Person], their differences are put to the test and one of them has to pull the trigger. Meanwhile, the rest of 15 Division is on the hunt for an armed burglar following a string of ATM robberies, and Dov is forced to work with rookie Chloe Price who he hooked up with at Frank and Noelle's engagement party. Together they track down the burglar, but Dov is wary of his new partner and her methods. Andy and Dov are both out to prove their history within 15 Division, but soon realize that there's value in having a fresh perspective while on the job.
After the stress of the last shift - where Swarek's new girlfriend Marlo had to shoot a suspect in order to save Andy's life - Andy just wants to make it through this shift and head out for a much-needed long weekend with her fellow rookies. Andy and Gail set out to work a routine highway spot check run by 15 Division in preparation for the holiday weekend. When the highway check leads to the discovery of a teenage boy, bound and gagged in the trunk of a car, Andy and Gail are plunged into a messy tale of young love gone very wrong. In an effort to save the boy from making a tragic mistake, Andy must convince him that no heartache lasts forever - if only she believed that herself.
When a beautiful woman staggers into the street and collapses from an apparent overdose, Andy and Marlo chase down her dealer, tackling him in a cloud of powdered heroin. The arrest sends 15 Division into emergency response mode when the drugs prove to be laced with deadly bacteria. Led by Swarek, the troops scramble to get the contaminated heroin off the streets before a deadly outbreak sweeps through the city. Andy and Marlo are plunged into an awkwardly intimate decontamination process, forcing Marlo to expose a shocking secret. Meanwhile, Dov and Chloe comfort a man whose wife has fallen victim to the dirty drugs, and their budding romance is threatened when Dov suspects Chloe of harboring a dark secret.
When the investigation of a missing girl is linked to a convicted murderer who has a past with 15 Division, it's all hands on deck. Detective Luke Callaghan comes in to help bring the copycat abductor to justice and bring the missing girl home safely. While most of the officers are out looking for witnesses, Luke and Chris discover that the imprisoned convict himself may prove to be a useful ally in the investigation. Meanwhile, Gail is forced to confront her worst fear in order to help solve the case, pushing her recent infidelity into the harsh light of day. Back at 15, Traci is assigned to Guns and Gangs Division and must work with Steve Peck. Her emotional response to this specific case pushes her to realize that she must let go of the past and try to get over Jerry Barber. Meanwhile, Swarek and Andy work together to locate the copycat - coming face-to-face with their unspoken past and the real reason for their breakup.
When a missing persons case is reopened, Swarek teams with his training officer to solve it; Chloe and Dov deal with a witch and her client.
It's the day of Frank and Noelle's wedding and everyone at 15 Division, especially Andy, is hoping for an easy shift so they can get out early and start celebrating. But those hopes are dashed when Andy and Nick discover a casino-bound bus full of old people who've just been robbed at gunpoint. The case gets messier as they realize they may be looking for an "inside man" and will have to re-interview all the seniors involved. Meanwhile, wedding preparations are also getting messy - in fact, the bride seems to have disappeared. Andy and Nick track down and stake out their bad guy, while Traci and Steve Peck find Noelle and try to convince her to tell Frank what's going on. Andy's hopes for a perfect day take a different turn when she and Nick wait for the bad guy and share a strangely intimate moment together.
Chris and his family return to the city for a visit. But Chris' reunion with his rookie family takes a shocking turn when his son, Christian, is abducted from a local park while under the care of Andy and Nick. It's all hands on deck at 15 Division in an effort to find Christian and his captor. As Swarek questions Andy and Nick about what happened at the playground, he inevitably uncovers the truth about their budding relationship. Meanwhile, Traci attempts to gather information from a devastated Denise as Chris struggles with being treated like a victim, and is kept in the dark about the investigation - unable to do any police work. Bringing Christian home safely can't spare Chris from a startling revelation that may tear his family apart..
It's nearing the end of shift at 15 Division when a routine grow-op bust leads to the discovery of a body. While Swarek, Oliver, Chris and Dov manage to escape work just in time - Andy, Chloe, Nick, Gail and Traci are roped into working overtime with the next platoon. Together they help manage what has become a rather unusual hug case - and Traci's first solo hugging case. In the meantime, Swarek, Oliver, Chris and Dov head off to Oliver's new cabin in the woods for a day of generator repairing and male bonding. While Traci bumps heads with Steve on the hug case, Andy accompanies Gail to the hospital for an injury that happened at the grow-op. Both Andy and Swarek have something to confess, but it might be too little too late for both of them.
Marlo reveals to Andy that she's been watching suspected pedophile Kevin Ford for days. When Andy answers a home invasion call and finds Ford badly beaten, she fears the investigation will point back to Marlo. She contacts Swarek, who is stunned by the news and scrambles to cover Marlo's tracks - recruiting Andy to help. Together, they keep the spotlight off Marlo long enough to find the assailant. But Swarek and Andy are left shaken by the day's events and the choices they've made. Meanwhile, Traci oversees a major drug sweep, with Chloe, Nick, Dov and Gail assisting. While 15 Division is poised to break the single week record for drug collars, Sam leaves his post with Traci to help Marlo, causing the operation to take an unexpected turn. Traci reaches out to Steve Peck for help, and together they attempt to capture a ruthless gang leader.
Andy and Chloe are blindsided when they respond to a routine 911 call and an unseen shooter gravely wounds Chloe. After shots are fired at a subsequent emergency call, the officers realize they are the targets of someone with a grudge against 15 Division. The team must continue to serve and protect the community while in the line of fire - and hopefully figure out who is hunting them before anyone else gets hurt. When Andy figures out that the shooter is Kevin Ford - the suspect that Marlo had been stalking in her manic state - Andy suspects that her transgression in helping Swarek cover for Marlo's actions may have come back to bite them all. Swarek and Andy realize that off-duty Marlo might also be in trouble and head off to find her before Ford can. Andy and Swarek find Marlo safe, but Oliver's arrival at Marlo's house brings him face-to-face with Kevin Ford.
Oliver Shaw is taken hostage and used as a decoy for Kevin Ford's ultimate plan to seek revenge on the officers at 15 Division - specifically those who were involved in questioning his involvement with the abduction of Chris Diaz's son. Many of 15's officers are sequestered to the station as a safety measure but Ford outsmarts them and in the end, shoots another officer. While the officers are dealing with the repercussions of Oliver's abduction, Dov and Wes argue over what course of action would serve Chloe best for her recovery, with Wes having the upper hand as Chloe's husband. In the end, the officers are reunited at the hospital, not knowing whether things will ever be the same.