Her first day begins disastrously; she was left to pursue two suspects on foot on her own as she had forgotten to turn her radio on and arrests them for drug possession, only to find out that one of them, Sam Swarek, was actually an undercover cop from her division and was two weeks away from a potential bust of a distribution ring – this was the premise for her love-hate relationship with him when he was assigned as her training officer.
The last three episodes of season 2 show viewers the beginning of Swarek and McNally's relationship, even though he is undercover, which leads them both to suspension for "conduct unbecoming" when they are discovered.
In the beginning of season 3, McNally returns from a trip after her three months' suspension and gets her job back as well as restarting her relationship with Swarek.
When her mother suddenly appears in her life due to a work case, she decides to let her back in after she asked for a second chance.
Described as the "dork" of the batch of rookies assigned to 15 Division, he was a sickly asthmatic kid (he has been seen using his inhaler a number of times in the first two seasons) who watched reruns of Starsky and Hutch.
In season 4 he has a one-night stand with Chloe Price, a woman he met in a bar and later finds out to be Frank's god-daughter as well as 15 Division's latest officer.
She comes from a family of cops; her mother is a superintendent, her father is an inspector, her brother Steve a successful narcotics detective, and her godfather is the chief of police.
Due to jealousy and unresolved feelings, she claims to have cheated on Nick with a detective who worked with him and Andy during their sting operation.
Following the events in "Ninety Degrees", she becomes estranged from her father after Steve confesses to his involvement in Commissioner Santana's corruption scandal to spare her from going against her conscience and lying in court.
Complications arise both at work and at home as she once dated Detective Jerry Barber until she decided to try to get back together with her son's father, Dex.
He is dedicated to his job and his friends and is extremely loyal to the point where he has risked his own life several times to save his colleagues.
After his parents' death, he lived with an uncle but ran away and was subsequently sent to a group home when he was caught stealing a car.
Sometime at the end of season 5, he meets Juliet Ward, an Internal Affairs officer undercover as a street cop placed in 15 Division, and starts to fall for her and pursues her.
After Nick confronts the driver involved in the accident that killed his parents and paralyzed his brother, he sleeps with Juliet, and the two appear to be sneaking around afterwards.
She appears for the first time in season 4, when she is first introduced as a one-night stand of Dov's and then later revealed to be Frank's goddaughter and the newest rookie to be transferred to 15 Division.
From Season 4 onwards, he begins opening up to Andy and his long-time friend Oliver Shaw, who are the only people he has confided in.
Swarek owns a goldendoodle named Boo Radley, whom he bought for McNally (as he promised after they previously broken up) before they get married.
[3] His father Jay Swarek (Nicholas Campbell) was an abusive alcoholic who was "in and out of prisons all our lives" and used to hit his wife and two children to the point where the police have been called.
He appears too laid back and is often seen eating a burger or sandwich in his car while on patrol waiting for a call but reveals that he did it so that his rookie would have the chance to take a breather and not rush into things.
In season 5, he is promoted to staff sergeant after Frank Best was removed in light of Swarek's shooting and because Ford managed to elude security and enter the premises with a weapon.
He is still a very hands-on cop and continues to be a mentor and confidant for his officers; for example, he personally led the search for a young autistic boy who had wandered off.
He was involved in a relationship with McNally, despite Swarek warning her that Callaghan had a reputation for dating one rookie a year, until he cheated on her with his ex-girlfriend, Jo Rosati.
McNally ends the relationship, but Luke wants her back, and the fallout causes Jo Rosati to leave 15 Division.
It is revealed in the season 2 episode "The One That Got Away", that the murder of 15 Division rookie Zoe Martinelli had been weighing heavily on his mind, which explains his determination to finding justice for the victims.
Detective Barber died in episode 9 of season 3 due to a fatal stab wound to the abdomen while trying to solve Peck's kidnapping.
Barber's death would affect 15 Division as a grief-stricken Nash struggles to move on while Swarek shuts down emotionally, leading to his break-up with McNally.
In season 4, she becomes engaged to Staff Sergeant Frank Best after he proposes to her following a faked homicide involving officers Peck and Epstein.
In episode 12 of season 4, Marlo's obsession with proving a man is a child molester causes her condition to take her over.
He gets ticked off with Andy when she and Chris arrest a drug dealer who is part of an operation that he is running on the dark, which makes him look like a fool.
He is partnered up with Traci to serve as a distraction from dealing with the copycat of the man that murdered Jerry, her dead fiance.