The episode kicks off with the rookies at their favorite bar, The Black Penny. Gail angrily observes Andy and Nick, while Dove jealously watches Chloe flirt with a firefighter. Gail thinks Nick is laughing too hard and making too-direct eye contact. In the midst of Gail's worried observations (that we all know are pretty well-founded), Dove goes to pull Chloe away from the firefighter. He asks her to go to a movie screening with him the next day, and that even though he's still worried about Staff Sergeant Frank Best being her godfather, it's worth the risk. Chloe seductively responds, "you have no idea," and says, as she walks away, "it's about time Dove Epstein."
In the next scene, Nick makes breakfast for Gail, but it includes eggs, which she doesn't like, and tomatoes, which she's allergic to. He says he got used to making the recipe while undercover and Gail sharply responds "why don't you wrap it up and bring it to Andy," to which Nick does not reply.
At the station, Andy tells Traci she wants to try to be friends with Marlo, whom she happens to be partnered with for the day. Then, as Chloe leaves the locker room, Dove enters and she drops her bag. After the encounter, Dove picks up something he thinks she missed. Andy identifies it as bipolar disorder medication, which freaks Dove out.
While on patrol, Dove and Chloe are called to a house, arriving as EMTs are putting a woman, Joelle, in an ambulance. The EMTs say that she overdosed on heroine but her husband, Mr. Bancroft, insists she doesn't do drugs. Then he says that her ex Nathan Barnes, a junkie, must have something to do with her overdose.
In the squad car, Marlo resists Andy's attempts to make conversation, saying she believes it's better for a good police partnership if they are not too close with one another. Andy asks about her and Sam, to which Marlo replies, "Sam and I work because we keep an emotional distance." At that, they get a call about Nathan Barnes. They arrive at his art gallery and find him nearly passed out in a pile of drugs. He tosses heroine all over the women in his attempt to flee, but they arrest him and call in emergency services.
Back at the hospital, the nurse tells Bancroft, Dove and Chloe that it's more serious than an overdose. Cut to Staff Sergeant Best informing Andy and Marlo, via radio, that the drugs are contaminated with Anthrax. Andy panics and is frustrated that Marlo acts so calm about it, saying she should drop the tough-guy, ice queen act.
At the station, Traci tells Frank and Sam that the anthrax is in the heroine. Jacob Blackstone from the drug squad is brought in to help with the case. Shaw is assigned to heading the effort to round up any users who've had the drug so they can get treated and the disease can be contained. Sam and Traci are assigned to finding the source of the contaminated heroine. Gail, with a mischievous look on her face, offers to help Blackstone with the administrative work.
Marlo tells Sam the drugs are stamped as Midnight. Howard, from hazmat, arrives at the art gallery and asks if Marlo and Andy are sisters.(Awkward...though they do look a little too similar, if you ask me).
Cut to Bancroft telling Traci that his wife had gone to Nathan's art opening. While they're waiting for Barnes or Mrs. Bancroft to awaken, Dove asks a nurse about bipolar disorder. Some of the symptoms she describes fits Chloe's personality—flight of ideas (racing thoughts, cluttered speech), which convinces Dove of his suspicions.
Back at the art gallery, Howard tells Andy and Marlo that they'll be fine after taking antibiotics to ward off the Anthrax, but that they need to remove their clothes to be decontaminated before leaving the building. (At this, all fans interested in the female sex became ecstatic, though were later disappointed, as ABC kept it PG).
Shoot to the station, where Nick has produced a file of all the recent drug arrests involving Midnight heroine. Sam and Nick look for a possible source in the area where most occurred—near Nathan's art gallery. When Gail and Blackstone are alone at the station, she asks him about Nick and Andy's relationship during their assignment. He says they got along well, and that even in their free time there was a rapport. A shocked Gail inquires about this 'free time', to which Blackstone responds that he gives undercover agents weekends off so that they can go home, but Andy and Nick chose to spend them together, doing things like bike rides up north. Upon seeing Gail's reaction, Blackstone realizes she's Nick's girlfriend and that he's said too much.
Meanwhile, Chris and Oliver enlist the help of Cedric, a junkie Chris knows, to find possible infected junkies.
Elsewhere, Sam follows up on a lead to the supplier of Midnight heroine. They talk to a man named Wendell, who's just the middleman, but he had sold to Nathan Barnes, and he gives Sam the name of the real supplier.
Cut back to Andy and Marlo. A hesitant Marlo asks Andy to tell her a secret. When Andy asks why, Marlo replies "So we're even," as she reveals the case of bipolar medication that the rookies had thought was Chloe's. Andy is silent but Marlo voices what she thinks Andy wants to say—how could she be a cop and have the disorder? Then, Marlo continues, insisting that she's managing her condition.
Meanwhile, Cedric leads Chris and Oliver to an infected junkie. Sam and Nick find out that the Midnight heroine they found at the real supplier's base a couple of scenes prior, does not contain Anthrax.
In the next scene, Marlo tells Andy that Frank doesn't know and she could be suspended for not disclosing the information. Andy asks if she could be fired even if tests prove she's managing the disorder. Marlo says no, but she could get stuck on desk duty for the rest of her career, and that everyone would forever pity and doubt her. Andy then says she's never doubted Marlo's ability to do the job, and she still doesn't.
At the police station, Traci informs Nick, Sam, Frank and Blackstone that all the victims all attended Nathan's art opening, which means Nathan's personal stash was probably spiked with the Anthrax. Blackstone says the only way someone outside the military could access the disease is from a natural outbreak, likely to occur in animals, to which Dove points out that Mr. Bancroft is a veterinarian. They bring Bancroft in and he admits that he'd responded to an outbreak of Anthrax on a farm, and saw an opportunity to take Nathan out of his wife's life, essentially confessing.
Later, in the locker room, Dove cancels his date with Chloe, telling her he 'knows' she's bipolar. An outraged Chloe denies it and says that he's using it as an out because he's scared of their relationship. Then, Marlo asks Andy not to tell Sam about her disorder.
Next, as Nick (flirtatiously) asks Andy how she is, Gail looks on with a sad-angry look that is becoming far too frequent on her. Chris tells Oliver he's leaving for Timmins the next day, thanking him for his guidance.
At The Black Penny, Dove and Gail are moping together again. Gail approaches Blackstone once more and asks him if something happened between Andy and Nick. He says he doesn't know, but that it doesn't matter because she's clearly decided something did. Then she asks him if he wants to leave the bar with her. (Come on really, this is her revenge? Lame choice Gail.)
In the closing moments, Andy is at home looking up information about bipolar disorder, and Chris gives a sad last look around 15 Division on his way out.