Deb’s monitoring of Dex leads to the death of a young girl.

Dexter is trapped. Deb is watching his every move and he can’t stand it. “Buck the System” begins with a couple of disturbing and entertaining murder fantasies Dexter has as he gets frustrated with the people around him.

But then he actually roughs up some suspect in custody and Deb flips. They discuss their predicament and Deb says she would need him to promise he wouldn’t do anything bad for her to stop watching him. “I can promise you that if you don’t get off my ass,” he replies, “something bad is going to happen.”

The camera looks down on the siblings as they stand in a narrow alleyway, both Morgans trapped by family ties and joint (albeit uneven) criminality. Deb appeals to Dexter’s fatherhood, saying of Harrison, “Sooner or later, this is going to catch up to you. And to him.”

Hasn’t it already? Dexter found Harrison crying in a pool of his mother’s blood in the fourth season finale, partly due to Dexter’s homicidal tendencies.

Deb agrees to give Dexter some space, and he instantly starts investigating his next kill, of course. And as part of his continuing torment of Louis, Dexter mails the at-large Ice Truck Killer hand to Masuka, who promptly fires his latest wayward intern. Poor Vince. Later, Dexter mails Jamie a video Louis made of himself with a hooker, so she breaks up with him. Louis sucks.

Meanwhile, Quinn and Nadia’s romance brews, but we find out she’s been ordered to inform on Miami Metro for the club owners, who are apparently in the heroin business. Most of this mob story line is unfortunately tired and cheesy, but what else would you expect from a Quinn storyline?

Deb, looking to fill Dexter’s days, orders him to accompany Batista to the house of Hannah McKay, former accomplice of last episode’s suicidal prisoner. Hannah sends strong sexy signals toward Dexter while being generally uncooperative. We can expect to see much more of her this season, which is great, because she’s strikingly beautiful and well-acted.

Batista comments on the need to investigate a case more than 6 years old: “Just when you think a case is closed, it pops open again.” Hey, like LaGuerta popping open the Bay Harbor Butcher case! I see what you did there.

The episode revolves around Dexter and Deb arguing about the proper course to take with Dexter’s latest quarry, a killer who escaped justice. Dexter is certain the man will soon kill again. Deb tells him he can’t prove the killer’s guilt or predict his actions. But he’s done it (mostly) well thus far, Deb. Just let him do his thing.

Presenting her with his personally gathered evidence, Dexter tells her, “An alarm is going off inside my lizard brain.” Deb, the surprisingly resilient comeback queen she is, responds, “Great. Now my brother has a lizard brain.”

Then she lays down the law: “Do not f— with me on this or I swear to God I will take us both down.” Is she bluffing?

At the marina, where Isaak and company have tracked the late Viktor Baskov’s last GPS whereabouts, Louis meddles with Dexter’s boat. Isaak shows up and nearly kills him, thinking he’s Viktor’s killer, before Louis cops to trespassing. He rats on Dexter and Isaak shoots him in the head anyway. Finally! Louis sucked.

The final act of the episode is one of the show’s creepiest and gut-wrenching in a long time. Dexter’s latest prey brings a bar girl back to his house and disgustingly seduces and inevitably attacks her.

Dexter, being a good brother, is off retrieving more evidence to prove the killer’s culpability to Deb. But Deb, on a hunch, is at the house already, watching. Her voice mail message doesn’t get through to Dex for a bit.

She sees strobe lights and hears heavy metal playing and breaks in to the house alone. For a few terrifying minutes, she searches around a junkyard-themed, Saw-esque second floor as music blares and lights flash. The killer is stalking the bar girl still — or is he? Hands reach out and pull Deb to the floor, where barbed wire snags her hand.

The killer stands over her, wearing a Nordic horned helmet and a chilling, dominating smile. And whack! Dexter saves the day by knocking him out. Dexter and Deb go check on the girl, who’s been killed in the same manner as the killer’s previous victims, as Dexter had concluded far earlier. Tires screech and the killer escapes. Of course.

Deb sees what happens when the law lets killers slip through the cracks and that her brother was right. In the latest example of the show’s burgeoning beauty and Deb enhancement, we see her stare out at the Atlantic, alone and reflective. Deb is absolutely the co-star of the show now and that’s fine. She’s excellent.

Back at Deb’s, she tells Dexter he was right. “I get it — what you do,” she tells him, “I hate it, but I get it.” She doesn’t accept it, but understands it as a necessary evil.

“But what I don’t understand are the blood slides,” Deb says, qualifying her reversal. She knows Dexter enjoys killing, deep down, and that he can’t change. She tells him she can’t change either. Unstoppable force, meet immovable object.

She tells him to move out, an implicit endorsement of his brand of justice. It’s a careful and well-constructed scene that doesn’t seem forced or inevitable. “I’m still your brother,” he tells her, “Nothing’s changed.”

Deb tells him everything’s changed and they can never go back to how they were.

The episode ends with Dexter himself staring out at the Atlantic, thinking, “I’m finally out of my cage. But freedom comes with a cost.” Will Deb send evil victims Dexter’s way? Will Dexter clear his kills with Deb before he acts on them? Will Deb even collaborate with Dexter?

Probably not. She wants to remove herself from the guilt as much as she can. But her permitting Dexter’s moving back into his apartment is a big step for her. But in what direction? Poor Deb.

“Buck the System” was filled with tension, from the sexual with Hannah to the gruesome with the girl-killer, and excitement, from Isaak’s gracious murdering of Louis to Deb’s solo jaunt through the terror maze. This week’s hour brought satisfying closure to the Louis storyline and opened a tantalizing Hannah plot, continuing the trend stellar performances this season. Also, at one point, Deb refers to Viktor as “Baskov F—wadinov.” Thank you, Deb, for existing.

Next week, we’ll see Isaak close in on Dexter (for real this time) and Dexter find his wayward prey.

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