Dexter's analysis of Zack's studio yields signs of a struggle – and some DNA evidence Zack planted, no doubt for Dexter to find. The DNA yields a familial match to Vogel, and when Dexter confronts her, she tells him she had two sons – until her older one "Daniel" killed her younger one and revealed himself as a psychopath, which is what brought her into her field. Unfortunately, she couldn't help her son, so she institutionalized him at age fourteen; three years later, he and several of the other children died in a fire. Dexter thinks the DNA evidence can only mean Daniel faked his death, and using age-projection software and a photo of the young Daniel, Dexter comes up with TBS – Oliver. He eventually confirms this result via a DNA sample from Oliver -- but when he takes it to Vogel, she begs Dexter not to kill him. Let's come back to that, shall we?
Hey, remember Hannah's trashy friend Arlene Schram? Turns out Hannah's been sending her money to hold for her to the tune of half a million bucks, and now she wants to collect. However, Elway brings in a U.S. Marshal on the Hannah case, and the guy goes straight to Dexter, thinking he might be a target for Hannah. Knowing that the marshal might catch her if she goes to see Arlene, Dexter takes her himself, and sure enough, the marshal shows up just moments after them -- so Dexter plays like Arlene's his main squeeze. This gets them out of their temporary jam, but Dexter tells Hannah that given the unis watching his apartment, there's only one private place he can think of to hole her up in – Deb's. Cue the Odd Couple theme song! Deb reluctantly lets Hannah stay, but everyone knows this is a stopgap solution, and eventually Hannah advances the idea of Argentina for real, and Dexter surprisingly agrees almost without hesitation.
Feeling unable to pursue him by himself (and he doesn't know the half of it), Quinn asks Deb to look into Zack for him, and Deb is chagrined to recall she just met him in Dexter and Hannah's company. She gets the story from Dexter, which only makes her bitterly realize there's no way she can return to the force while Dexter is there. Somehow, she ends up talking to Jamie about the Cassie case on Quinn's behalf, and Jamie lets some things about Oliver be known that raise red flags for Deb. She and Quinn go to ask him some more questions under the guise of trying to catch the killer, and he's actually not too great at fielding Deb's inquiries. When Deb mentions to Dexter that she's on to him, though, he begs her to give him a wide berth, but even though she agrees, obviously Deb is aware enough of Oliver as a killer that that has to come into play. And speaking of which, let's finish what we started…
…which is that Dexter indulges Vogel while secretly still planning to kill Oliver, and the two of them conspire to draw Oliver out. The plan as far as Vogel knows is for her to meet Oliver at a diner that holds meaning from his past, but Dexter drugs her and goes to the place himself. Of course, Dexter underestimates the guy by a power of one million, and he escapes. Dexter then goes to Vogel, but she tells him she can never trust him again – which should really cut both ways, given that she's reconciled with her son. As was said in several different contexts this episode, this can't end well.
Dexter and Hannah are in bed, apparently having managed to secure the same hotel room despite the fact that Hannah surely checked out. Well, I suppose they didn't really need maid service. Dexter laments the fact that he has to go, whereupon Hannah tells him she's so sorry about Zack. Me too! If nothing else, he represented an age group not actually seen too often on this show. On the subject of his dead protégé, Dexter says he has to go see Vogel so they can figure out their next move before telling Hannah to remain indoors and out of sight. Hannah, however, has an errand she can't ignore: she's got to go see Arlene, who's holding some money Hannah sent her. It's money she's going to need now that Maggie Castner's bank accounts are closed to her. Dexter agrees and starts to say something else, but Hannah tells him she knows – be careful. Dexter: "Actually, I was gonna say 'I love you'." I only, like, seventy percent believe him, but still, Dexter FTW! Maggie returns the sentiment, and he buries his face in her neck…
…and then we cut to Vogel sighing her sympathy over Zack's fate and her realization that she'll have to figure out some way to let the family know. However you phrase it, Vogel, I doubt this is going to help the mother's drinking problem any. A wider shot reveals that they're in Zack's studio, and Dexter asks for a little time before she starts the process that will bring in MM. He then plays the song (which is a new method for me in scoring the episode title) but while Vogel tells him she's been listening to it for ages, she can't ascribe any significance to it other than supposing some of her patients might have heard her playing it over the years.
With that seemingly a dead end, Dexter, having concluded from the positioning of the furniture that a struggle took place, does a quick analysis but concludes that the killer cleaned up after himself. Vogel desperately says that TBS must be trying to send her a message rather than merely scaring her to death, and I'm not sure I completely follow that logic, not that Dexter cares either way: TBS killed Zack and then dumped his body in his house. "What if Harrison had been home?" I'm guessing he would have ended up with more bloody stuffed animals. Dexter declares his intention to kill TBS, and then further investigation bears fruit -- on the underside of one of Zack's desks -- where he finds a few hairs and some blood. It's hard to tell but he might have stuck them to some adhesive, maybe even gum? Whatever the case, it looks evident that Zack managed to leave a DNA sample of his killer as his last act. Vogel admires how calm and rational he obviously summoned the wherewithal to stay, an emotional evenness she's not sure Dexter's matching.