Rookie Blue has never been afraid to give its characters flaws, and we see several of them hitting bottom or close to it in this week's episode. While some seem to be on the mend and others are still on the edge of disaster, it's installments like "Going Under" that remind us why we've followed these people for five seasons: we can identify with them and their foibles as much as their strengths. They might no longer be rookies by definition, but sometimes they certainly still act like it.

As we learned last week, Chris Diaz has a drug problem. Now everyone else is starting to notice, beginning with his partner of the week, Dov Epstein, who wants to know why Diaz is falling asleep on the job and getting philosophical on him. Before he can question Diaz too thoroughly, though, they're called to a fistfight at a fraternity house. Unfortunately, Diaz is the one who punches one of the suspects a few times. That's definitely ill-advised behavior.

The next morning, Diaz meets with his dealer, to whom he owes a considerable amount of money. He then reports to 15 Division, where Oliver is discussing the gun that Dov pulled off one of the suspects from the night before. Apparently, it came from a local bar, and the team is going undercover to shut down the illegal firearm-sales operation taking place therein.

And for some reason, Duncan is still here, causing Andy to make her sad face. "Walk away," Nick advises her, but then Oliver tells her that Duncan and his lawyer just made "serious allegiations against this division and against you." While an investigation takes place, Andy will be under a microscope, while Duncan remains a presence in the office, and she's advised to avoid him. An annoyed Andy seeks out Sam, who offers her "the workplace equivalent of pity sex": the missing-persons case he's working on.

Traci and Steve are in charge of the undercover operation, which means they're running Diaz, Dov and Nick. While Nick settles at the bar and gets hit on by the bartender, the other two lurk elsewhere, keeping an eye on the other, less attractive bartender that is their main suspect. Diaz approaches him and tries to make a gun deal. Dov is not impressed with his partner's undercover skills, but Diaz assures him that everything's going to be great. That is, until he spots his drug dealer in the same bar.

Naturally, the dealer has seen Diaz, too - and when Diaz goes out to get into position for the gun bust, there's a confrontation that ends with Diaz arresting the dealer and screwing up the undercover gig, much to everyone's frustration. Once the dealer finds out that Diaz is a cop, he's willing to use his dirty little secret as leverage. "The first name I'm going to give up is yours," he threatens.

Back at the office, Oliver wonders if Gail has a screw loose somewhere after the death of the young mother she tried to save, and Sam and Andy track their missing person's money in an attempt to find out where he's gone. "This guy's life reads like the most boring novel in the world, except for this," Sam opines, pointing out the man's repeated and expensive trips to Costa Rica. When Andy discovers a letter from a college in Costa Rica, the case seems pretty much closed.

More importantly, Sam is there to try and help Andy with her Duncan problem. She admits to him that she previously left things out of her reports in an attempt to help out Duncan, and is worried about those omissions being discovered. He jokingly suggests they retire to Costa Rica.