With summer drawing to an end, tensions begin to rise. Seth continues to be stubborn in his decision to not return home from Oregon where he spent the summer. In Chino, Ryan continues to work his construction job while living with a pregnant Theresa and missing what he left behind. Back in Newport, Summer deals with Seth's disappearance and Marissa continues to distance herself from her new life, finding comfort in her old habits.

During the first day of school for the gang, Seth and Ryan continue to try to win back the affection of Summer and Marissa. Hailey informs Jimmy about her decision to move away from Newport and Jimmy makes a drastic decision in order to stay with her. Meanwhile, Sandy explains Caleb's legal troubles to Julie, who proceeds to threaten Sandy.

Seth, with the help of Alex, tries to prove that he's different that what everyone thinks of him. Summer debates her feelings for Seth now that Zach has entered the picture. Marissa and Ryan try to mend their relationship, but when Ryan meets Lindsay, it makes things a little harder. Meanwhile, Caleb's legal troubles may leave both Sandy and Kirsten unemployed and Julie plans her course of action if Caleb is taken into custody.

Ryan and Seth go on an awkward double date at a Killers' concert, and end up leaving with someone other than who they came with. Meanwhile, Julie finds her new position at the Newport Group harder than she expected and turns to Kirsten for relief.

With the arrival of winter, Seth, Ryan, Summer and Marissa find themselves planning for the annual SnO.C.Winter Ball. Julie and Jimmy continue spending time together and getting along surprisingly well. Meanwhile, Sandy comes closer to finding the truth about Caleb's secret and Julie uncovers what Marissa has been hiding.

Everyone is invited to the annual Chrismukkah celebration at the Cohens, but Seth's holiday is tested when secrets are exposed that may change everyone's life forever. Meanwhile, Summer hatches a plan to save Chrismukkah.

The Cohens decide to welcome Lindsay into the family after Caleb's secret was revealed. Another family is shattered when news is revealed that one of them is leaving Newport. Meanwhile, Seth attempts to be a bad boy to impress Alex.

Lindsay and Ryan struggle to define their relationship: Is Lindsay technically Ryan's aunt or is it okay for her to be his girlfriend? Meanwhile, Summer's meeting with Zach's family goes horribly, as does Sandy and Kirsten's anniversary until Sandy finds a way to come through at the end. Marissa deals with being alone when D.J. leaves Newport.

Marissa and Summer plan a girls' night out and decide to invite Lindsay to go out with them. While the girls are out, Ryan, Seth, and Zach spend the evening playing video games, but have a hard time not joining the girls. Meanwhile, Julie and Kirsten brainstorm ideas to help the Newport Group's image while Sandy and Caleb try to talk them out of their idea.

Zach and Seth launch their own comic book series based on their lives. During their initial sketches, Summer finds that she's being excluded from the book and asks the boys why. Sandy agrees to help an old friend find his long-lost daughter, but the search only raises questions. Meanwhile, Marissa rebels against Caleb; and Ryan agrees to help Lindsay find out more about Caleb.

Summer takes a more active role in Seth and Zach's comic book, which makes Seth uncomfortable since he'll now be forced to spend time alone with her. Sandy and Rebecca talk about their past together and Kirsten talks to Caleb about him having a relationship with Lindsay. Later, Caleb decides what kind of relationship he wants with both of his daughters, and Alex confronts Marissa about where their relationship is headed.

Sandy attempts to make up for hiding Rebecca's return from Kirsten by planning a romantic Valentine's Day. Zach, Seth and Summer travel to San Diego to meet with a company about their comic book, but Seth focuses more on getting Summer back then pitching the comic book. Meanwhile, Julie returns from her vacation determined to reconnect with Marissa, and Ryan and Lindsay try to spend Valentine's Day together, but her relationship with Caleb interferes.

While Rebecca is mourning the death of her father, Kirsten becomes increasingly impatient with Sandy's absence, leading her to visit Rebecca. Meanwhile, Marissa hangs out with Alex's friends and Caleb seeks proof that Lindsay is his daughter.

It's raining in Newport, and the rain begins to have its affect on the O.C. Marissa tells Julie about her relationship with Alex, then informs Julie that she's moving in with Alex. Summer gets ready for her trip to Italy with Zach, but she has a hard time forgetting about Seth. After persuasion from Ryan, Lindsay agrees to take the paternity test and everyone awaits the results. Meanwhile, after the highway is closed due to rain, Sandy and Rebecca are stranded together in a motel.

Marissa begins to see the reality of living on her own, and takes a break from the housework to go to the mall with Summer, Seth, and Ryan. While waiting the arrival of Carter, the new editor for Newport Living, Julie is distracted when a man from her past resurfaces. Meanwhile, Sandy and Caleb find themselves bonding over their experiences.

Seth sets out on a mission to bring back the fun that the gang had last year, and won't stop until his plan is a success. Alex becomes jealous when Marissa asks Ryan for help for an upcoming school bonfire. Meanwhile, Kirsten has to talk Carter in to staying with the magazine and Sandy helps Julie with her blackmailer problem.

When Ryan's brother comes to visit, Ryan has a feeling that it won't turn out well, but the Cohens gladly welcome Trey into their household. Marissa and Ryan continue to build on their new friendship while Seth and Summer deal with unexpected news. Meanwhile, Julie tells Kirsten about her past indiscretions then turns to Caleb for help.

Sandy agrees to be the honorary chair for the Newport Beach charity yard sale and enlists help from Trey. But when the most expensive item at the sale goes missing, Ryan and Seth set out to retrieve it. Meanwhile, Trey looks for somewhere more permanent to live and the Cohens get yet another house guest.

As Trey's birthday rolls around, Marissa throws him a party, which quickly escalates into a dangerous rager. Sandy and Carter go surfing together, and after meeting a girl on the beach, they decide to double-date. Summer uncovers Reed's true identity, getting Seth in trouble. Meanwhile, Julie leaves town due to her recent embarrassment, but ends up coming home and reconnecting with an old friend.

In the aftermath of the rager, Sandy, Ryan and Seth go undercover to save Trey from jail time. Kirsten and Carter take a trip to wine country for wine sampling, and they become increasingly close. Meanwhile, Caleb returns from his Europe trip and ignores Julie's advances.

Sandy, Seth and Ryan travel to South Beach for a visit with the Nana, who is now living in Miami Beach. While playing shuffleboard, Seth and Ryan meet a girl who gets Seth on stage for Music Video Nation's Spring Break broadcast. Seth gets himself in hot water when Summer sees Seth with another girl on TV. Meanwhile, Carter decides to leave Newport Living and Trey reads too much into Marissa's friendship.

Now that Carter left Newport, Kirsten has started drinking more than usual, and refuses to admit to Sandy that she has a problem. Ryan begins to suspect that something happened between Trey and Marissa, but isn't sure what occurred. Summer confronts Reed about Seth and Zach's graphic novel, and ends up making an appearance at the Atomic County launch party. Meanwhile, Caleb confronts Julie about her past indiscretions.

Sandy and Caleb team up to confront Kirsten about her recent drinking problem. Ryan, determined to find out what happened between Trey and Marissa, heads to Chino for answers. Summer decides to not let Zach and Seth fighting over her ruin her prom, and forces the boys to choose who will escort her to the dance. Meanwhile, Julie and Caleb celebrate their final evening together before the divorce is finalized.

Sandy is forced to make a difficult, life-changing decision to save Kirsten and their marriage. Seth, Summer, Marissa, and Ryan are caught in the middle of a fight involving Trey at The Bait Shop, and when Ryan finds out what happened between Trey and Marissa, gunfire breaks out. Meanwhile, Jimmy, worried about Marissa and his family, returns to Newport.