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.
Now that Ryan and Seth have returned to Newport, its time to get back to school. The boys reunite with their former girlfriends but it becomes apparent that things will not be going back to the way they were. Caleb's legal troubles catch up with him, and Hailey has bad news for Jimmy.
After Caleb is released from custody, he must make some decisions about the Newport Group, and his decisions don't sit well with Kirsten. Seth is determined to win back Summer's friendship, even if it means getting a job. Ryan has a string of awkward encounters with a new girl at school, while Marissa faces being alone.
It's a new era in Newport Beach. Kirsten and Julie try and work together at The Newport Group, while Sandy tries to find out what Caleb is hiding. Seth and Ryan go on a double date with Lindsay and Alex, but it ends differently than the four of them might have expected.
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.
It's Chrismukkah once again in The O.C. and this year, Seth wants to take it global. As Caleb is faced with the looming prospect of going to prison, Sandy convinces him to come forward and reveal the truth about his involvement with Renee. Julie and Jimmy spend some quality time together. Caleb's confession shocks the Cohen and Nichol family to the core, and nothing will be the same again.
After Caleb's confession, The Cohens try to welcome Lindsay as part of the family. Jimmy is faced with a choice when Marissa finds out about his relationship with his ex-wife, Julie. Seth attempts to be a "bad boy" like Ryan to impress Alex. Summer finds out what Zach did on his Christmas break while Ryan and Lindsay want to know if it is too awkward for them to be together.
When Sandy forgets his and Kirsten's 20th wedding anniversary, he finds himself going to a lot of trouble to make it up to her. Seth and Ryan's respective relationships with Alex and Lindsay add to the trouble, and doubt falls upon their futures together. Marissa and Julie continue to butt heads over DJ, and Summer wants to take her relationship with Zach to the next level.
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.
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.
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.
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.
After Lindsay moves to Chicago with her mom, Ryan is heartbroken and Caleb's confused. Seth, Summer, and Marissa invite Ryan to the mall to get his mind off things. Caleb tags along to vent when Sandy tries to make amends with Kirsten by searching for her wedding ring or even a grand gesture that will take her back to when Sandy first proposed. Meanwhile, Alex learns that Marissa might not be as into her as she thought she would be. Kirsten and Julie call an editor for Newport Living but at the same time Julie's Riverside past catches up to her.
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 Ryan's brother, Trey, gets out of jail and gives Ryan a call, the Cohens give him a warm welcome. Though, Ryan knows nothing good can come from it. Zach returns from his Italy trip and gives some news leaving Summer curious and Seth ... not really sure what to think. Meanwhile, after Julie admits her naked mistake to Kirsten, they both agree to get Caleb in on it for more help. Newport Living is officially launched.
Sandy reluctantly serves as host/auctioneer at yet another OC benefit; Trey lies to Ryan to prove that he can be self-dependent; Carter reinvigorates Seth and Zach's comic book dream, much to the annoyance of Summer. Marissa moves in with the Cohens.
It's Trey's 21st birthday and Marissa, anxious to help Trey and Ryan forge a relationship, throws a birthday party for Trey at the Cooper-Nichol mansion that turns out to be a rager. Seth is left in hot water by both Summer and Zach, but why? Sandy wants to continue his friendship with Carter, leaving a confused Kirsten. Meanwhile, Julie shares some memories with an "old" friend and in the process learns a few things and gains a few ideas.
Sandy and the boys devise a plan to save Trey from going to jail after he took the fall for supplying drugs at his birthday party. Kirsten and Carter, fulfilling more than their editorial duties, take a trip to Featherbrook's winery. Meanwhile, Caleb returns home and questions his marriage with Julie.
Sandy, Seth and Ryan travel to Miami to visit the Nana after she tells Sandy she is getting married. Unable to go because of work, Kirsten is left on her own to deal with her feelings for Carter. Trey misinterprets Marissa's friendship; Zach invites Summer to a home-cooked dinner.
Seth and Zach declare themselves rivals for Summer's affections; Marissa is still reeling from being nearly raped by Trey; Sandy confronts Kirsten when her drinking problem becomes very apparent; Caleb hands divorce papers to Julie.
Seth and Zach squabble over who should take Summer to the prom and who will meet George Lucas for dinner; Sandy takes drastic actions with a recovering Kirsten; Ryan starts to piece together what happened between Trey and Marissa, courtesy of Theresa; Julie's attempts to poison Caleb take a surprise turn.
Kirsten hits rock bottom after Caleb's funeral, prompting Sandy to finally take action; Jimmy Cooper resurfaces and almost immediately renews ties with a mourning Julie; Jess bullies Trey into participating in a drug deal that turns into a shootout; Ryan finally learns the truth about what happened between Trey and Marissa, leading to a bloody confrontation.
