As the gang all continue to deal with Marissa's passing, they all begin to adjust to life after high school. At Brown, Summer has re-invented herself into an environmental activist with help of her new friend, Che. Ryan has moved out of the Cohen household and continues to cope Marissa's passing, which he took the hardest of all. But when Ryan misses a dinner at the Cohens, Seth finds Ryan slipping back to his old ways and calls Summer for help. Meanwhile, Julie finds it hard to deal with her daughter's passing; Kaitlin uses the Cooper charm to get a new pair of boots; and Taylor tries to avoid everyone in order to hide the fact that she's not in Paris.

Ryan and Seth leave the O.C. to go south of the border, each to find something they're looking for -- Ryan searches for Volchok in order to deal with his emotions and Seth has an encounter with the U.S. Marines. Meanwhile, Julie and Kaitlin are forced by the Dean to work together on the Harbor school clothing drive and Julie is shut out by the Cohens when they discover what she's up to. Elsewhere, Taylor springs a surprise visit on Summer who continues to distance herself from California.

It's Thanksgiving in the O.C. as the Cohen family continues to deal with the happenings in Mexico. Sandy is able to cut a deal to help Volchok, but ends up letting Ryan intervene, allowing him to settle the score. At Brown, as Summer packs up to come back to Seth and the new Roberts family, she wonders if she'll be able to deal with Newport. Meanwhile, Julie has trouble dealing with both Kaitlin Dr. Roberts, leading her to spend most of her Thanksgiving alone.

Summer tries to return to the popular, gossipy girl that she was in high school, but finds that her time at Brown may have changed who she is forever. Despite her changes, Seth continues to support the new Summer. Meanwhile, Ryan helps Taylor out of a predicament and Julie and Kaitlin try to stay out of trouble, but find it hard to do.

Taylor, after learning about Ryan's insomnia, comes up with her own therapeutic remedy to help him sleep. At Brown, Summer and Che step up their activism and their actions lead to more freedom than they had imagined. Meanwhile, Kaitlin helps Julie save the dating service and Taylor is forced to turn to Kaitlin for help with Ryan when her plan backfires.

Seth travels to Providence to visit Summer, but when he gets to Brown all he finds is an empty dorm room. While Seth is away, Kirsten and Sandy are able to find an opportunity to bond with Ryan, who's in a haze after Taylor's doings. Meanwhile, Kaitlin takes the school mean girl down the best way she knows how -- a rager at the Roberts' house. Elsewhere, Julie finds that Newmatch is more than a dating service and the truth about Che is finally exposed.

It's once again Chrismukkah in the O.C. and it's up to Taylor and Ryan to make Chrismukkah as it should be. After Ryan and Taylor fall of the roof of the Cohen's house, Ryan falls unconscious. While in the hospital, Ryan dreams of how life would be if he never came to Newport. With Taylor's help, Ryan is able to get life back to the way that it was before the accident.

It's New Year's Eve in the O.C. and to celebrate the holiday, Ryan puts together a road trip as a surprise for Taylor, but his surprise is spoiled when Summer finds out that Seth hasn't planned anything for her. Later, the four head to Las Vegas where they meet a raver who may be able to help Summer's future. Back in Newport, the Bullit has a friend look into Julie's accounting.

When Kaitlin fails a class presentation, she's paired up with Will, the Harbor band geek. Kaitlin thinks that she can get Will to do her work for her, but when he begins to challenge her, sparks between the two emerge. After their new commitment from New Year's Eve, Seth and Summer begin making plans but fail to recognize their true feelings. Meanwhile, Sandy confronts Frank and Julie reveals Frank's true identity to Kirsten.

Henri-Michel, Taylor's ex-husband, arrives in the O.C. to promote his erotic memoir of his times with Taylor. When Henri-Michel and Ryan meet and Ryan learns of Taylor's time in France with Henri-Michel, Ryan begins to second guess his relationship with Taylor. Seth visits Dr. Roberts in order to ask for Summer's hand in marriage while Summer visits Holly who has a vision of Summer's future. Meanwhile, Kaitlin realizes that she has feelings for Will.

Seth and Che take a trip together to the forest. There, Che has a dream that tells him that his spirit is in love with an unexpected someone. After the trip, Seth and Summer are finally able to speak to one another. Meanwhile, Julie finds a way back into Kirsten's life, Julie-style, Kaitlin shuts out Will, and Taylor decides that she has to stop speaking to Ryan.

As Kirsten's 40th birthday is nearing, Ryan and the Cohen men prepare a party for her while Kirsten receives news that will forever change her life. Meanwhile, Julie has a secret love affair that Kaitlin tries to stop; and Taylor visits a therapist who gives her advice while Ryan pretends not to miss her. Elsewhere, when the plan to save Newport Chuck falls through, Che finally learns that Seth is not his soulmate.

Taylor concocts a plan to make things right between Ryan and his father. Julie continues to further her relationship with Bullit but when Julie and Frank run into each other in front of Ryan and Taylor it becomes apparent that Julie is having a secret affair with Frank. When Kaitlin finds out that Ryan and Taylor are working to get Julie and Frank together, she goes out of her way to make her mother like Bullit, forcing Julie to choose between Frank and Bullit. Meanwhile, Summer begins to question if Seth is the one for her after she and Seth get a psychic reading.

Taylor and Ryan finally say those three words to each other, but did they both mean it? Summer challenges Seth to find something he's passionate about. Kirsten, for the first time in her life, has a realization about what Newport is really all about. Meanwhile, Kaitlin continues to attempt to break up her mother's relationship with Frank, but Julie begins to notice Kaitlin's efforts and ends up making a change. As the night comes to a close, Newport is shaken up by an earthquake.

After the earthquake, Newport residents scramble to make sure everyone is alright. Sandy tells Seth to say put, but Seth does whatever he can to be with Ryan and Taylor. When Seth and Ryan finally meet back up, Seth begins to realize what it means to be a brother. While Summer and Taylor can't find Pancakes and begin frantically searching for him, Taylor sees that bonds people have with their family are greater than she could have ever imagined. Meanwhile, Julie and Kaitlin are stuck in an ice cream shop until the only man for Julie arrives to save them. But everything isn't as it seems when everyone meets up at the Cohen house.

Newport was forever changed with the arrival of outside Ryan Atwood four years ago. In the series finale, an emotional ending shows what happens to each of the lives that Ryan touched, including the Cohens, Coopers, and his most recent love, Taylor.