It's a new year, and the year is 1986. That's only technically, though, because this show has very nearly forgotten it takes place in the 1980s, except that Carrie is on a pay phone instead of a cell phone. She's struggling to get a hold of Sebastian because this is before the times of internet and the Snapchat.
Mouse is sad, having just broken up with West. She also feels weird now that she has gotten into Harvard and has no goals left to achieve. Donna LeDonna is feeling the same way, having realized she has dated everyone eligible at Castlebury High. The two purposeless gals spot Dorrit flirting with some alternative looking guy and smiling a lot, so Donna investigates further. She gives Dorrit the advice to catch this bad boy with "a casual laidback situation" like a party to get some extracurricular time in.
Back in the city, Larissa is newly and obnoxiously engaged to Harlan. Larissa loves her giant ring more than she loves Harlan, and is bored with Carrie's tales of the "challenges" of a long-distance long-term relationship. She advises Carrie not to let one person move for the other, as it builds pressure and resentment.
Tom brings Carrie and Dorrit into the kitchen to tell them he's been assigned to a high-profile, confidential, big-deal case this weekend, which means he'll be gone all weekend and the girls have the house to themselves. Tom tells them it's a test, and doesn't explicitly say "no parties" because it's not like this is an '80s movie. It's a TV show. Carrie invites Sebastian over for the weekend to have the whole house to themselves while Dorrit plans a big party.
Sebastian, it turns out, can't be available for this episode on such short notice. Carrie was hoping for some big romantic gesture but sometimes this show tries to imitate real life. But not this time, of course! Sebastian flew in at the last moment to spend the weekend with Carrie. Dorrit invites the boy of her dreams and his random friend to the "rager" at her house.
Tom's big-deal merger turns out to just be negotiations between Harlan and Larissa. They want a pre-nup from Tom, even though he doesn't do that kind of law and he's a friend. Harlan makes a deal to pay Tom double his hourly rate.
Carrie and Sebastian make out on her bed and she lovingly asks him how long he can stay. Sebastian tells her he's there for good, that he's moving back and even though he was only in Laguna for a month or something, Carrie is like "uh oh." Larissa really got her in her head. There's no time for further making out, though, because the party is starting downstairs and that is not a sexual metaphor.