In the season three opener, we start off with senior year starting in a few days. It has been a couple of months since the shooting. Trey has been in a coma all this time leaving the D.A. impatiently waiting for his side of the story. Feeling the pressure from the community the D.A. continues to put pressure on Ryan, Marisa, Seth, Summer and others for more information. Meanwhile Julie is steaming mad because Caleb’s accountant won’t give up the goods on Caleb’s will and Kirsten is still in rehab.

Social Chair Marissa tries to plan this year’s carnival but is having much difficulty due to a little pest named Taylor. Sandy is surprised to find Kirsten has been released from rehab but instead of informing him, she has been living with another woman named Charlotte who she met in rehab. Julie and Sandy meet with the Dean and Dr. Kim to discuss Ryan and Marissa’s future at Harbor High. Later Ryan and Marissa partake in a similar meeting. Elsewhere Jimmy meets a man and feels more than open to discuss his money problems with him.

Jimmy must make once more the difficult decision to leave or stay with his family; Kirsten returns to the Cohen household while Charlotte has to leave her house; Marissa and Ryan face a possible separation; Taylor lords over the school musical and makes Summer and Seth her stooges.

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.

Ryan decides to make a life-changing decision that Sandy reluctantly supports; Marissa looks to her new friends to help with her latest crisis with Ryan; Summer and Seth go on a mission to reveal a secret between Taylor and Dean Hess; Charlotte makes Julie an offer that she can't refuse.

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.

Ryan, Summer, Seth, and Marissa think about what they'd like to do with their lives as they compile their college applications, but they run across several obstacles; Johnny's surfing career takes an unfortunate twist; a revealed secret complicates Julie's friendship with Kirsten.

Marissa visits Johnny at his house after his accident to see how he is doing. Julie and Kirsten get a contract for their new catering business while Sandy, Ryan and Matt meet with investors.

Chrismukkah has returned to the Cohen house and Seth suggests to have a Bar Mitzvah to Ryan to raise money for Johnny's knee surgery after his accident but Ryan then pulls out after seeing Johnny and Marissa get too close.

Ryan, Seth and Summer launch a campaign to get Marissa back into Harbour. With the help of Dr. Kim they get the board of trustees to hear the case will Marissa be permitted to return for her final semester?

It's winter break and Kaitlin Cooper has returned home from her Montecito boarding school.

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.

Sandy and Matt's hospital project hits a roadblock. Kirsten and Julie run into troubles of their own with their matchmaking business. Marissa confesses her feelings to Johnny. Summer finds out out about Seth.

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.

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.