Graduation: A time for friends, family, celebration , remembrance, letting go, and moving on to tomorrow. Odd man out in all the events is Volchok. Then, it happens...

Sandy's picture will be on the cover of Riviera. Or, if the DA has his way, a mugshot. More troubles: Volchok blackmails Ryan into a crime, Kirsten is drinking and Seth burns down his dad's office. Didn't mean to. Honest.

Right dress. Wrong date. If you can't go to prom with the person you like, go with someone who makes the person you like jealous. There'll be many memories for the Core 4 after this prom. Not all of them will be happy.

Some go East, some stay West for freshman orientation weekends. Ryan and Marissa arrive at Berkley, and Summer and student/poseur Seth visit Brown. Familiar faces also surface. Anna is at Brown. And doesn't Theresa's baby look like... Ryan?

No party, no surf. It's a different Dawn Patrol: Ryan's off to Albuquerque to invite his mom Dawn to graduation (maybe). Seth quits Summer (oops). And, at Volchok's party, Marissa sees something secretly poured into a drink (uh-oh).

The Sweatshirt Bonfire is the party where all the seniors wear the sweatshirt of the college they'll be attending. Hey, Seth's not wearing a sweatshirt. What's up with that?

A race to the bottom: Marissa's ties with Volchok may send her into substance-abuse freefall... but not before she gathers herself enough to save the Ryan-Sadie match. The Newport Group events turn violent.

Trouble's at the door. It's Jess, Trey's sometime girlfriend, and she's coming on to Ryan like Little Miss Vulnerable. Seth and Summer discover a, uh, wheelbarrow. And all those burning Volchok-Marissa glances ignite a bonfire.

A little Cohen arm-twisting convinces Ryan that, yeah, he should do something special for his 18th birthday. So who's he going to invite to the party at The Bait Shop? Sadie? For sure. Marissa? Well...

Ryan road-trips with Sadie. Strip poker, anyone? Summer learns - ew! - about her dad and Julie. Kirsten worries that The Newport Group is corrupting Sandy. And Marissa is a suspect in Johnny's death.

Life goes on. Love goes on. But how can anyone focus on Valentine's Day after Johnny's tragedy? Well, maybe love and understanding are the medicines everyone needs now. Even so, nothing will be easy.

The Johnny-Kaitlin-Marissa-Ryan situation finds closure, perhaps, and tragedy for sure. Summer may be running out of forgiveness for Seth's half-truths. And Dr. Roberts makes a startling admission to Julie.

Little sister. Big problems. Kaitlin feels she's spent her whole life in Marissa's shadow - maybe now it's time for some payback. Seth, meanwhile, may be seeing his college plans go up in smoke.

Mini-Cooper isn't so mini now. Kaitlin, Marissa's younger sister, returns to O.C. - and she carries lots of emotional baggage. Plus: Kirsten-Julie dating biz is blackmailed into setting up a date for Veronica.

One semester to go. Wouldn't it go better if Marissa returned to Harbor High? Seth, Ryan, Summer and even Taylor campaign for her readmittance - and Johnny steps out of the way. For now.

Seth's a man with a holiday plan: Hold an honorary bar mitzvah for Ryan and donate the funds to Johnny's surgery. Now try to convince Ryan to do it... and persuade Johnny to accept it. Also: Julie + Summer's dad? Stay tuned!

Maybe Seth should accompany Summer to college. She's the one who (surprise!) aces the SAT. Another surprise: Sandy's right-hand man jeopardizes a Newport Group account by hanging out at a strip club.

Time to think about the future - college applications are due. Seth's eager, especially if the school is as far from Newport as possible. Will Summer go with him? Marissa has another plan: She won't apply.

Divide and conquer. Taylor schemes to separate Summer and Seth. Volchok tries to goad Ryan into a fight. Julie upends Charlotte's charity scam.

Party by night. Surf by first light. The Dawn Patrol beach blast is the backdrop for a developing Marissa-Johnny attraction. Meanwhile, Taylor has a crush on... Seth?! And Ryan again lets his fists do his talking.

Is Ryan shipping out? Sandy's mind-meld persuasion skills fail to convince him to stay. But where there's a will, there's another Cohen way. Charlotte's now zeroes in on Julie; Summer outwits Taylor.

Public school? No problem. Marissa's glad to be at a school where no one knows about her. Oops. Everyone knows. Charlotte closes in on Kirsten. And Summer spies someone lip-locking with Taylor.

Ryan. Marissa. A romantic beach hut. You do the math. Meanwhile, the math in Caleb's will is easy: Julie gets zilch, Kirsten gets nada. Caleb was broke and that adds up to a new dynamic for the Cohens, Coopers and The Newport Group.

Senior year. But not for everyone. Marissa is expelled. Julie scrambles for cash, Sandy scrambles to make sense of Kirsten's rehab and, to top it all, Ryan loses his cool and gets himself expelled, too.

Trey's in a coma, Marissa pulled the trigger, who's going to take the fall? Ryan? But maybe there's a way out for Kid Chino.