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.

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.

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 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.

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.

Marissa struggles with attending a public high school, and her new friends make Ryan jealous; Charlotte's ruse is up when she learns that Kirsten did not inherit any money from Caleb; Julie's mansion is repossessed, leaving her with nothing; Taylor's power play dethrones Summer as commitee chairman, who then learns an interesting secret about her and Dean Hess.

When the gang heads to The Bait Shop to see The Subways, Taylor's horsing around causes a major rift between Seth and Summer. Marissa, determined to end the potentially explosive rift between Ryan and Volchok, does whatever it takes to watch out for Ryan. Meanwhile, Matt, Sandy's Newport Group partner, goes above and beyond the call of duty when he learns he has to cut Newport Group staff.

Kaitlin starts going out with Johnny with the sole motive of making Marissa jealous. Seth starts getting nostalgic as he prepares for his Brown interview.

Ryan thinks about not inviting Marissa to his 18th birthday after their falling out but Sandy tries to get him to invite a relative. After months of avoiding telling Summer and Marissa the truth Neil and Julie tell Summer and Marissa the truth.

Harbor School's Senior Prom brings out the best and the worst in everyone. Taylor hits a high at prom, while Summer falls flat with her surprising prom date. Seth and Anna come up with a prom plan to win Summer back. Meanwhile, Marissa convinces Volchok to go to the prom and Ryan brings an unexpected old flame. When one of the girls is elected prom queen, it truly becomes a night to remember. Later, when Taylor's prom funds are missing, Ryan returns to his old ways and prom night turns brutal.

Ryan and Seth go to Mexico to find Volchok, but Seth secretly phones home worried about what Ryan might do if they find him. A distraught Sandy and Kirsten decide to go after the boys and try to stop Ryan before he does something he might regret. Meanwhile, Julie and Kaitlin are forced to participate in a clothing drive at the school, and Summer and her hippie friends try to save a tree.

When Taylor learns about Ryan's sleeping problems she devises a plan to help him but must then turn to Kaitlin when her plan backfires. At Brown Summer and Che step up their activism and it lands them more freedom than imagined. Julie also gets help from Kaitlin to help save the dating service.

Chrismukkah once again returns to the Cohens. Ryan and Taylor have an accident while putting up the holiday lights and they end up in a sort of parallel universe, where all is not like it is meant to be, since Ryan does not exist. While - in the real world - everybody is worrying about them, except Taylor's mom, they eventually manage to fix things, also thanks to a letter from the past.

It's Taylor's birthday and she hopes Ryan will tell her he loves her. She's unsure whether she should tell him she got into Berkeley as well. Summer feels that Seth should find something in his life that he really cares about whereas Kirsten is having second thoughts about raising another child in Newport. Then the unexpected happens even though it's been predicted.