Dexter, in LaGuerta's office, starts rambling about how better understanding the science behind meteorology could have untold benefits in his crime scene analysis, on and on, yammering enthusiastically. Finally, when he stops to take a breath, LaGuerta's like, "If I let you go, will you stop talking?" But Dexter doesn't stop talking, in fact he goes back to speedily making his case, at which point the POV switches from LaGuerta to Rita, who also waits for Dexter to run out of steam before giving Dex a confused look and a shrugged-shoulder acquiescence. Man, that couples therapy has done wonders for the shady crap Rita will once again let Dexter pull. She does barter with him, however; in exchange for taking off this weekend, Dexter can cover for her during her Tuesday night yoga classes. She also mentions that next-door neighbor (and recently single, if you recall last week's unsubtle breadcrumb-dropping) Elliott is taking the kids fishing on Saturday. Dex doesn't give much of an indication one way or the other whether this bothers him, but he's been steadily more proprietary over his family this season, so we can guess.
We see Dexter packing a bag for his trip, including his murderin' tools as well as the bludgeoning hammer Arthur gave him. Meanwhile, Arthur is also packing, which includes portioning out some of his sister's ashes into a travel vial. Kind of like you do with your contact lens solution.
The next morning, Dexter scares the crap out of Arthur as he's preparing to leave school for Tampa. He asks if he can grab a ride to Tampa and help on the build. Arthur declines and starts to drive off, but "Kyle" is increasingly desperate, talking about how he needs to get out of here, and out of here (pointing to his head). To a less murderously unhinged man, Dexter's words and demeanor here would be setting off every alarm bell you have. But with Arthur, Dexter's appeals to his charitable posturing (he references "generosity of spirit") and messianic tendencies ("you're the only one who can help me") eventually wear him down. He lets Dexter into the passenger seat and drives off.