Overall, it felt a bit like a filler, with nothing really interesting happening. In a nutshell, Dexter gets to the top of the love triangle, with Hannah and Debra on the other sides. We also learn the true origins of Isaak's excessive desire to avenge Viktor's death.
It starts with Dex and Hannah having breakfast after the night they spent together, just like any normal couple. The calendar on Hannah's wall with a picture of Argentina leads to a conversation where she explains why this country has a special meaning for her - it feels like home.
After that Dex leaves his safe place and tries to convince his sister that killing Hannah won't do any good to her, he says she wouldn't be able to be at peace with herself, should he proceed. Debra doesn't seem to agree, but he puts his foot down.
So to make her feel better, Dex goes to the doughnut shop and is assassinated there by Sirko, who managed to get rid of his police tail. Dexter obviously survives and after telling his sister about the incident, he gets the carte blanche from her to deal with the crazy avenger. However, turns out that Dexter has a competitor: George gets an approval from his Ukrainian bosses to get rid of Sirko, since his little vendetta puts Koshka's drug business in jeopardy. Around the same time Dexter gets a message from his mother-in-law that Rita's father is having an operation, so Astor, Cody and Harrison have to go to Miami, but since it's too dangerous to be in Dexter's apartment with Isaak on the loose, Debra agrees to let the kids stay in her place instead. Dex also tells Hannah about Isaak, and she gives him an advice: he must gather more information about the guy. She gives him her van so he could follow Sirko in a vehicle no one knows is his.
And so the police, including Dexter, comes to investigate this murder. Dexter briefly describes what happened and leaves to spend some time with the kids on the beach. Hanna comes there as well, to get one of the keys she gave Dex together with her car keys and learn that he has a son and two step-children. She says she'd want to meet them someday.
A little later Dexter comes to check on how Debra is doing with the kids and she recognizes the key holder. She asks if he's sleeping with Hannah, he confesses he has feelings for her. Debra freaks out and in her hysteria lets out that she was (maybe still is) in love with him.
After being kicked out from Deb's apartment, Dexter follows Isaak to a gay bar, where he learns that Baskov was Isaak's great love.
That's pretty much it, from other events, Quinn had to work for George again, Batista opened his restaurant, Astor smokes pot and Maria learns that Dexter moved his boat when the police learnt about Bay Harbor Butcher existence.