It's New Years and "love" is in the air. A sister returns home, and throws a party. Oliver casts his spell. Sandy and Kirsten find themselves in a rut, and out of their environment. Seth finally makes a decision between Summer and Anna. 2004 is born.

Hailey discovers she isn't as welcome as she might have thought. Perhaps the Cohen house is full? Luke makes some new friends, while Seth and Anna try to tell a friend about their decision. Rooney sings, a lot, and Oliver noses, a lot. Trouble brews.

Oliver plans a trip for the gang, but things don't go as well as everyone might have hoped- except for Oliver of course. Summer really plants herself into "friendship" within Anna and Seth's relationship. Jimmy and Sandy decide to go into business together. Caleb and Julie return from Europe, and Hailey and Kirsten butt heads. Ryan finds himself in the cold.

It's Valentine's Day in Newport, and everyone is preparing for the Valentine's singles benefit. Luke has a new crush, while Sandy gives Seth some sex advice. An old friend of Ryan's shows up in town, which forces Marissa to lay her feelings on the table. Seth and Summer take their relationship to another level.

When Summer and Marissa run into Grady Bridges, the star of Summer's favourite prime-time soap 'The Valley', he invites them to an LA party, and they accept. On the road trip to LA, Seth, Summer, Marissa, and Ryan run into Hailey, now working at a strip club. Jimmy and Sandy present their restaurant to Newport's finest and approach Caleb for some financial help.

Nana Cohen comes to California for Passover with some sad news. Ryan searches for Marissa, who'd gone missing after learning Julie and Luke's secret, and he finds her in the most unlikely of places. Summer tries to impress Nana. Jimmy and Hailey take a step forward.

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.

Marissa plots to ruin her mother's happiness by inviting someone from her mother's past to the wedding shower, while Sandy attempts to hide Theresa's return to the O.C. from Ryan. When Ryan discovers Sandy's lies and Theresa's reasons for coming back, he puts himself in a dangerous situation. In the meantime, Seth's unimpressive first meeting with Summer's father leaves their relationship in doubt.

It's Bachelor and Bachelorette party time for Caleb and Julie. Caleb and the boys head off to Las Vegas while Kirsten organizes an evening at the Cohen house for Julie, strippers included. Seth and Ryan get themselves into a tricky situation in Vegas while back home, Summer pines over lost love, and Marissa comforts Theresa. Jimmy and Sandy learn of Caleb's true motives. Julie and Hailey take a swim, while Theresa has some shocking news for Ryan and Marissa.

Julie and Caleb walk down the aisle, as Ryan and Theresa face a difficult decision regarding their pregnancy. Marissa moves into Caleb and Julie's new castle on the cliff, and Seth struggles to find himself when things around him seem to fall apart. Life in The O.C. comes full circle from one year before.

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.

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.