Harlee's fierce loyalty - to her daughter and her family in blue - is put to the test when she's entrapped by the FBI.
Harlee struggles with becoming an FBI informant while Wozniak suspects every member of his crew. Paranoid that Wozniak's search will lead to her, Harlee suggests polygraphing the entire crew, a plan that backfires. An appeals case sends Wozniak to watch tapes from his first encounter with Harlee, revealing a dark secret from their past. Meanwhile, Espada helps Tess deal with her husband's infidelity.
Harlee refuses to give Stahl incriminating evidence against Wozniak but hands over a fully resolved unofficial investigation that Tess and Espada initiated when they moved a body by Tess' house found to be murdered with a 3D printed gun. Wozniak confronts Harlee who executes a dangerous bluff.
Loman's guilt over the death of an unarmed suspect becomes a liability for the crew, forcing them to test his loyalty. Wozniak sends Harlee on an errand that reveals he has a silent partner in the big score. Wozniak realizes Donnie has been withholding information. Harlee feigns cooperation with Stahl, but unwittingly leads the FBI to incriminating evidence.
Harlee and Tess intercept a heroin shipment from a local drug lord who discovers Wozniak's secret and tries to use it as leverage to get his confiscated drugs back. Harlee fears Stahl's interest in her goes beyond the professional, believing that he is invading her privacy, and resorts to spying on him with the help of an old friend. Tess tries to use Espada to get back at her husband.
Wozniak's suspicions land on Saperstein, forcing Harlee to convince Stahl to arrest Saperstein in order to keep him safe. Harlee visits her abusive ex, Miguel, who tasks her with tracking down the man who committed the murder for which he is imprisoned.
Wozniak grapples with his guilt over attempting to kill Saperstein as he is faced with the need to finish the job. Harlee desperately tries to protect Saperstein as she struggles to keep the unit from being torn apart by suspicion and grief. Stahl gets closer to uncovering Harlee's relationship with Miguel.
After Saperstein's death, Harlee realizes there are no lines Wozniak won't cross and decides to help Stahl's investigation. The crew retaliates against a pair of abusive cops after they harass Harlee's daughter Cristina and her boyfriend. Wozniak makes Loman feel like a part of the unit.
Harlee's collaboration with Stahl (Warrren Kole) uncovers Wozniak's secret relationship with Donnie. Wozniak decides to take charge of the big job after picking up a package that turns out to be a kidnapped man. A mysterious call to Saperstein's desk forces the crew to face their mixed feelings about their fallen friend and ultimately decide to preserve his memory.
Harlee and Wozniak secure more details on the big score and meet Donnie's silent partner, who reveals himself to be a DEA agent. They learn the big score is an armored truck heist. Wozniak takes interest in a fellow marine veteran who is suspected of murder under dubious circumstances.
On the day of the heist, Harlee realizes she misplaced her trust in Stahl but it's too late to back down. Harlee tries to get immunity for her crew as she participates in the heist knowing that it will lead to Wozniak's arrest. Miguel discovers Harlee's secret. Loman unwittingly witnesses the heist just as the exchange goes terribly wrong.
Harlee and Wozniak must face the consequences of the disappearance of the heist's money. Out of options, Harlee approaches the FBI and promises to find the money and deliver Wozniak and his partners if Stahl keeps her in play.
Unable to provide Donnie and Linklater to the FBI, Harlee makes a deal with Stahl to exchange her own immunity for that of her crew. Harlee comes face to face with Miguel who reveals his darkest side as he demands to be a part of Cristina's life.