At the end of last season, Ryan returned to Chino to protect his pregnant ex-girlfriend from her abusive ex-boyfriend and, with the departure of his only friend in Newport Beach, Seth decided to do what he always had planned: sail off on his yacht, the Summer Breeze, and run away from Newport Beach. With Seth and Ryan gone, Kirsten and Sandy try to stay busy with a remodeling project at home, while Julie and Caleb settle into their gorgeous Newport Beach mansion. But now the summer is over, school's about to begin and Ryan and Seth are nowhere to be found.

Ryan, Marissa, Seth and Summer are about to begin their senior year at Harbor High. But while everyone else celebrates the last days of summer, a dark cloud hangs over the foursome. Trey, Ryan's brother, still clings to life two months after the shooting and the cops want to know who really shot him. Meanwhile, Julie Cooper and Jimmy are determined to protect Marissa at whatever cost. And while Sandy, Seth and Ryan try to fend for themselves at home, Kirsten, still recovering in her plush rehab by the sea, forms a friendship with fellow recovering alcoholic Charlotte, who like all things in the O.C. may not be everything she appears to be.

High school seems like a distant memory to the young graduates, who have been thrust into adulthood after the death of their beloved friend. The Cohens struggle to keep Ryan focused on the future, fighting the forces that threaten to pull him back into his old life. Summer has left Orange County and is attending Brown University in Rhode Island, where she has reinvented herself as a left-leaning, tree-hugging activist. Meanwhile, Seth is lonely in the O.C. and working at a local comic book store. Kaitlin Cooper returns to the O.C. as the No. 1 troublemaker at The Harbor School. And Taylor Townsend is hiding out and speaking French in the O.C. after her mysterious romance in Paris.