While responding to a case, Hailey recognizes a man from when she was undercover in a different unit.
The case resurfaces some ghosts from her past namely Booth, the leader of the distribution ring who was recently released from prison on good behavior.
Hailey offers to revive Kelly, her old identity, and go back undercover. Voight is hesitant but he allows it under one condition -- she's not going to get revenge for her former partner and possible love interest Garrett, she's going solely for the drug bust.
Immediately, Hailey makes it known that this is more personal.
She makes her move during Scotty's funeral, standing far enough so she attracts Booth's attention.
When he approaches, she lays the groundwork by telling him she's interested in getting back into the game.
Booth insists he's in the paint business, but we all know that's really code for liquid meth.
He ambushes Kelly at her apartment to check her story out and offer her a small job for starters. She obliges and when she completes the task, asks if he'll meet with her buyer Ryan, Halstead's alias.
He thinks there's something super suspicious about her so he makes her answer personal questions about herself hoping to catch her in a lie.
Kelly doesn't slip up and passes the test, for now.
Meanwhile, Halstead wants to know just how dangerous Booth really is and what happened on New Year's Eve, since everyone keeps bringing it up.
He meets up with her old case officer who fills him in -- Kelly thought she could get the name of a major player from Booth while was drunk but instead was brutally beaten and almost raped.
The only reason she made it out alive was thanks to Garrett, who helped her get away and took her to a hospital.
Halstead wants her to pull out of the case but she assures him she's fine; she needs to take him down.
The next day she arrives at his factory, sneaks into his office and installs a camera just as he walks in through the door and reprimands her for being in his space.
Halstead, who is watching the situation unravel from an undercover van nearby, distracts Booth by calling Kelly's phone.
The two arrange a meeting and while Halstead thinks she's going overboard by making him memorize everything about his undercover alias, she tells him Booth will interrogate him.
When they meet up, Booth takes Ryan to the basement where he questions him about school mascots and if he's ever slept with Kelly.
Disgusted, Ryan refuses to answer and Booth tells him the deal is off because he "doesn't trust him."
Halstead tells Voight that Booth has some sick obsession with Hailey so he decides to pull the plug on the whole case.
But that doesn't sit well with Hailey who needs to book this guy and get her revenge.
Ignoring Voight's orders, she goes to Booth's office off-the-books and tries to salvage the deal.
Booth agrees only after she promises to have dinner with him after it's over and assures him that nothing is going on between her and Ryan because he has a wife and daughter.
Halstead gets a bad feeling after Hailey fails to pick up the phone and arrives and Booth's office to back her up.
Booth is furious but when he shows him the cash for the buy, he shuts up and agrees to meet later that night.
During the exchange, Booth stalls in order to grossly feel up Kelly for "wires." He still doesn't trust her.
Halstead gets uncomfortable and when he reaches for his pocket, the other guy assumes he has a gun.
Hailey outmaneuvers Booth and shoots his nephew dead. Then she pursues Booth and they get into a nasty fight in the staircase.
When Halstead finally arrives to help her, Hailey has Booth pinned down and is contemplating shooting him dead but decides not to.
Later that evening, when Hailey fails to show up at Molly's, Halstead comes to her place to check on her.
She insists that she's fine but you know she isn't. Eventually, she admits that revenge didn't feel as good as she thought it would.
Voight briefly meets with Olinksy to tell him that a grand jury is siding with Woods. He promises to turn himself in instead of letting Olinsky take the fall for Bingham's murder but Olinsky tells him he knew what he was getting into.