The summer is coming to an end, and Seth hasn't returned home to Newport. In the midst of home renovations, Sandy and Kirsten are strained and missing their son. Ryan has begun his new life with Theresa in Chino, as Marissa is alone and in a dark place. Summer is slowly getting over Seth.

It's Valentines Day 2005 in O.C. when Sandy and Kirsten's conflict about Rebecca continues. Ryan tries to make peace with Caleb to spend Valentines Day with Lindsay. While Seth, Zach, and Summer head to San Diego in hope to make their Comic Book idea succeed Seth has a breakdown and ends up in a lonely state. Marissa and Alex take a step forward in their relationship while Julie returns from her trip in hope to reconnect with Marissa.

It's the annual "SnO.C." winter dance. Seth wants to take his relationship with Alex further, but she might have other plans. Marissa's secret relationship with D.J. is exposed, while Zach continues his pursuit of a Seth-free relationship with Summer. Sandy solves Caleb's legal puzzle ending with a shocking twist.

Harbor School's Senior Prom brings out the best and the worst in everyone. Taylor hits a high at prom, while Summer falls flat with her surprising prom date. Seth and Anna come up with a prom plan to win Summer back. Meanwhile, Marissa convinces Volchok to go to the prom and Ryan brings an unexpected old flame. When one of the girls is elected prom queen, it truly becomes a night to remember. Later, when Taylor's prom funds are missing, Ryan returns to his old ways and prom night turns brutal.

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.

Jimmy and Sandy run into a roadblock in their effort to re-open the Balboa Lighthouse restaurant. Summer and Seth take it upon themselves to finally make Marissa feel more at home at Jimmy's. Everyone is shocked when Caleb makes a proposal, which sparks a near-fatal accident for Luke. Marissa, and Sandy and Jimmy each strike deals with Caleb. An old O.C. friend moves away.

After hearing about Lindsay and Ryan's failed attempt at dinner with Caleb the night before, Kirsten arranges a dinner that she can supervise. But her presence makes no difference when Ryan and Caleb butt heads and disaster strikes. Sandy keeps Rebecca's presence in Newport a secret from Kirsten after deciding to take her case to court. Marissa and Alex's relationship intensifies, while Seth and Summer spend a lot of time together at Zach's insistence.

Jimmy must make once more the difficult decision to leave or stay with his family; Kirsten returns to the Cohen household while Charlotte has to leave her house; Marissa and Ryan face a possible separation; Taylor lords over the school musical and makes Summer and Seth her stooges.

Seth and Kirsten want to avoid facing the world after their respective displays at Thanksgiving. Sandy has some advice for Jimmy, which doesn't sit well with Julie. Ryan and Luke find themselves getting along after an O.C. loved-one comes out of the closet to face negative reactions.

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 overcomes his indecision and competes with Henri-Michel for Taylor's affections. Che leads Seth on a trek of spiritual renewal. Meanwhile, Kirsten and Julie have to do some damage control.

It's time for the annual Cotillion ball. Cherished by many, hated by most, it's the time when Newport's finest young ladies enter society. More information about Jimmy's business trouble comes to light, and Kirsten's loan to Jimmy is not happy news for Sandy.

Sandy and Jimmy have plans to sign papers for the Lighthouse... but is Jimmy ready? Oliver makes his plans official to Ryan, but Ryan just cannot catch a break from anyone and finds himself in an all too familiar position. Summer has a "big" new boyfriend... for a little while. And Julie gets a job.

As Sandy continues working on Rebecca's case, Kirsten questions how strong their feelings are for one another. Seth continues his downward spiral in the aftermath of Zach and Summer's Valentine's night together, while Marissa confronts her fear about her relationship with Alex. Caleb offers Lindsay a place in his family, but not without a paternity test.

It's a rainy day in Newport where Lindsay decides to take the DNA test and makes a difficult choice regarding her life with Caleb while Ryan helps her through it. Sandy still wants to help Rebecca with her case and tries to convince her not to run again. Kirsten is still upset and wishes her marriage would go back to normal and for Rebecca's case to be over with. Meanwhile, Seth thinks he's lost Summer forever untill Summer is unable to hide from her feelings once and for all. Marissa moves out of the Cooper-Nichol mansion and moves in with Alex after she tells her mother about Alex being her girlfriend.

Spring has flung in Newport where Marissa recruits Ryan to help her with Harbor School's Pep Rally Bonfire. Alex accepts that her and Marissa are from two different lives and knows she is fighting a lost cause. Seth is stuck and misses last year's good times while Summer disagrees and tells him it's time to move on. Meanwhile, Julie still doesn't know what she is gonna do about her blackmailer and seeks some advice from Sandy. Kirsten wants Carter to stick with the magazine despite his reservations.

When an old friend returns to Newport, Ryan decides to help her, which causes problems for him and Sadie. When Summer and Seth run into another rough patch Taylor tries to help them out. Sandy has to deal with more problems with the Newport Group and Marissa must deal with being alone.

Julie and Dr. Roberts announce their engagement and the Cooper ladies move in with Summer and her dad. Meanwhile, Marissa drifts away from the group and Summer when she seeks solace in the bad company of Volchok and his crew. Ryan tries to move on with Sadie, but Seth's concerns for Marissa get in the way. At the Newport Group, Sandy's woes with Matt continue and Kirsten surprises Seth with a night out.

Kirsten's father, Caleb Nichol, comes for a visit along with his new girlfriend, Gabrielle. 24-year-old Grandma Nichol has arrived. Kirsten finds her job in jeopardy, while Sandy considers making a change. Much to Ryan's disappointment, Luke decides he wants a second chance with Marissa... but who will Marissa choose? Summer makes the moves on Seth, who wonders if her feelings are real.

Seth is thrown when Alex's ex arrives in town for a visit, and is even more thrown when he finds out who it is. Julie has a plan to try and save The Newport Group, which to everyone's surprise, might actually work. Lindsay sticks out when she joins Summer and Marissa for a girls night out; meanwhile, Ryan, Seth and Zach contemplate playing it safe on a guys' night in. But nothing ever goes as planned.

Things heat up between Sandy and Kirsten when Sandy decides to help his mentor in law school find his daughter, Rebecca, whom was Sandy's first love. Seth and Zach start up a comic book based on their lives in Newport, while Summer questions why she's been left out. Ryan makes the attempt to unite Lindsay and Caleb after Lindsay tells Ryan she is curious in getting to know her "new" father. While Julie is in Paris, Marissa looks for new ways to rebel; in doing so, her friendship with Alex increases.

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.

Seth and Summer come upon a fortune teller whose prediction makes the two question whether they are truly meant for each other. Ryan gets back in touch with his father (with a little help from Taylor). When the two teens discover a love connection between Frank and Julie despite her pending engagement, a Valentine's competition is sparked between "Team Bullit" and "Team Frank". Meanwhile, with a new baby on the way, Kirsten finds herself dwelling on an old secret involving Jimmy Cooper.

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.