Ryan Atwood, a teen from the wrong side of the tracks, moves in with a wealthy family willing to give him a chance. But Ryan's arrival disturbs the status quo of the affluent, privileged community of Newport Beach, California.
Seth works at a comic book store in Newport. Kirsten drops by the store to ask Seth if he has something to send Ryan in a "care package" she is preparing. Later, Seth tries to talk Ryan into coming to dinner at the Cohens' but the latter isn't easily persuaded, saying he works nights, etc. At her dorm, Summer and a friend named Che try to rally students for a protest. Summer gets a phone call from Taylor who seems to be in Paris.
Ryan and Seth go to Mexico to find Volchok, but Seth secretly phones home worried about what Ryan might do if they find him. A distraught Sandy and Kirsten decide to go after the boys and try to stop Ryan before he does something he might regret. Meanwhile, Julie and Kaitlin are forced to participate in a clothing drive at the school, and Summer and her hippie friends try to save a tree.
It's Thanksgiving time. Volchok is back in town and Ryan seems eager to take revenge. At first Sandy and Kirsten try to talk him out of this but, then, Sandy makes Ryan face Volchok. Julie's behavior turns Kaitlin away and permanently jeopardizes her relationship with Dr. Roberts. Summer starts protesting against poverty issues and turns the Cohens' Thanksgiving dinner into a homelesses' refuge. It's clear she hasn't got over Marissa's death yet. Veronica kicks Taylor out as soon as she discovers her beloved daughter married a French man during her European trip.
When Seth visits Summer, she realizes that her materialistic, gossiping self may be forever gone ever since she went to Brown. Ryan helps Taylor with her divorce. Temptation stirs when Kaitlin and Julie vow to stay out of trouble. And Sandy longs for a guy's night out.
When Taylor learns about Ryan's sleeping problems she devises a plan to help him but must then turn to Kaitlin when her plan backfires. At Brown Summer and Che step up their activism and it lands them more freedom than imagined. Julie also gets help from Kaitlin to help save the dating service.
Kaitlin takes down the school mean girl by throwing a rager at her house. Sandy and Kirsten find time to bond with Ryan while Seth is away visiting Summer at Brown but all he finds is an empty dorm room. Meanwhile Julie continues work on the dating service and finds out a new surprise.
Chrismukkah once again returns to the Cohens. Ryan and Taylor have an accident while putting up the holiday lights and they end up in a sort of parallel universe, where all is not like it is meant to be, since Ryan does not exist. While - in the real world - everybody is worrying about them, except Taylor's mom, they eventually manage to fix things, also thanks to a letter from the past.
Ryan and Taylor may be heading out on a Las Vegas road trip for New Year's Eve, but there's even more fun in store when Seth and Summer join them. Meanwhile Julie doesn't trust NewMatch's new accountant, and Sandy suspects the reason why.
Kaitlin is assigned a tutor after she presented a ruffled report to her class. Julie comes clean to Kirsten about who Frank really is. Summer and Seth continue their recent commitment despite how they really feel about it. When Ryan's dad comes to the Cohen's for dinner things don't go as planned.
Taylor's ex-husband comes to Newport and causes a rift in her relationship with Ryan. Seth goes to Seattle to obtain Dr. Roberts blessing, while Summer "travels to the past" for 48 hours. Kaitlin finds love, while Julie finds problems with Kirsten.
Kirsten is forced to clean up after Julie after one of the men she set up with a date experiences a medical condition. Will and his fellow band members have a bake sale so they can raise money to buy new uniforms.
Sandy, Ryan and Seth prepare for Kirsten's upcoming 40th birthday party, however Kirsten learns some news that will change her life forever. Kaitlin tries to stop her mother having a secret love affair. Taylor visits a therapist and Ryan pretends that he doesnt miss her. Elsewhere Che finally learns that Seth isn't his soulmate.
Taylor makes a plan to fix things between Ryan and his dad. Julie continues her relationship with Bullit but when she runs into Frank when Ryan and Taylor are around they realize that she has been having a secret affair with Frank. When Kaitlin learns that Taylor and Ryan are trying to break up Frank and Julie she tries to get her mother to like Bullit, which forces Julie to choose between Bullit and Frank.Summer begins to think about if Seth really is the one for her.
It's Taylor's birthday and she hopes Ryan will tell her he loves her. She's unsure whether she should tell him she got into Berkeley as well. Summer feels that Seth should find something in his life that he really cares about whereas Kirsten is having second thoughts about raising another child in Newport. Then the unexpected happens even though it's been predicted.
After the earthquake the residents of Newport scramble to make sure that everyone is ok. When Sandy speaks to Seth he tells him to stay put but he refuses as all he wants to do is be with Ryan and Taylor and Seth finds out what it is really like to be a brother.Taylor finally learns that the bonds between family are stronger than she thought. Julie and Kaitlin remain stuck in the Ice-Cream store before the only man for Julie turns up and saves them. They think everything is going well before they meet up at the Cohen's house and learn things are not what they seem.
The Cohen family says goodbye in the series finale as the episode jumps six months after the earthquake to show the Cohens, Ryan, Taylor, Summer and the Cooper women trying to piece their lives back together. The Cohens find out that fixing up their old mansion would cost more than rebuilding it but Ryan and Seth see this as an opportunity to give Sandy and an expecting Kirsten a surprise that'll change their lives.