Marissa discovers something in Johnny that Ryan lacks and Charlotte convinces Julie and Kirsten to throw a charity event. After discovering fake IDs in Charlotte's purse, Julie confronts her.

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 prepares herself to head back to school. Ryan has doubts about his relationship with Marissa after helping out Sadie. Kirsten and Julie make plans for a Valentine's Day party.

When Ryan and Sadie go on a road trip to search for one of Johnny's relatives they end up in Ryan's old neighborhood in Chino, which brings the two closer together. While the two are away Marissa is implicated in Johnny's death and Seth and Summer learn the truth about Julie and Dr. Roberts new secret.

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.

When an old friend returns to Newport, Ryan decides to help her, which causes problems for him and Sadie. When Summer and Seth run into another rough patch Taylor tries to help them out. Sandy has to deal with more problems with the Newport Group and Marissa must deal with being alone.

Julie and Dr. Roberts announce their engagement and the Cooper ladies move in with Summer and her dad. Meanwhile, Marissa drifts away from the group and Summer when she seeks solace in the bad company of Volchok and his crew. Ryan tries to move on with Sadie, but Seth's concerns for Marissa get in the way. At the Newport Group, Sandy's woes with Matt continue and Kirsten surprises Seth with a night out.

When college acceptance and rejection letters are received the kids must consider what path they are going to follow. Ryan and Sadie are trying to decide what they will do after high school. Taylor holds a college party and Summer and Marissa fix their friendship problems.

Ryan decides to go and invite his mother to his graduation party. Taylor and Summer work together to try and find out what Seth is hiding, but Summer ends up getting hurt. Marissa realises she is in bad company after she finds out how low Volchok's friends can go. Sandy is able to fix his work problems with Matt but while doing so he hurts Kirsten.

Graduation is just weeks away and Ryan, Seth, Summer and Marissa leave Newport to attend their respective pre-frosh orientations at UC Berkeley and Brown. While dropping the boys off at the airport, Kirsten is surprised to see Theresa and her all-too-familiar-looking baby boy. While visiting Brown, Seth runs into Anna on the quad and they instantly connect. Could this be the end of Summer and Seth? Meanwhile, Kirsten and Sandy attend a hospital benefit where Kirsten vents her frustrations and falls back into an old habit.

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 Volchok make a deal after a fallout at prom. Sandy is chosen as Man of the Year, but this sparks the DA to dig deeper into his involvement with the hospital problems. Marissa goes and sees Kaitlin at her boarding school and Seth visits the Newport Group HQ and lights up the joint.

It is graduation day for Ryan, Seth, Summer, and Marissa at Harbor High School. After graduation they all went out to eat and celebrate their graduation. Ryan's mother gave him a new car as a graduation present. Marissa is going to spend a year on a boat with her father as her graduation gift. Seth finally got into RISD for second semester. Summer is still going to Brown University in the fall. Volchok is still mad at Marissa and Ryan so he decided to crash Ryan's car.

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.

It's Thanksgiving time. Volchok is back in town and Ryan seems eager to take revenge. At first Sandy and Kirsten try to talk him out of this but, then, Sandy makes Ryan face Volchok. Julie's behavior turns Kaitlin away and permanently jeopardizes her relationship with Dr. Roberts. Summer starts protesting against poverty issues and turns the Cohens' Thanksgiving dinner into a homelesses' refuge. It's clear she hasn't got over Marissa's death yet. Veronica kicks Taylor out as soon as she discovers her beloved daughter married a French man during her European trip.