As Ryan, Marissa, Seth and Summer prepare for their senior year, Trey's shooting looms over them. Trey is clinging to life and when the cops continue to pressure the teens to admit who shot him, Ryan debates taking the blame to save Marissa. Meanwhile, Julie and Jimmy are determined to protect Marissa, no matter the cost, and Kirsten continues to recover in rehab with Charlotte, who is hiding her ulterior motives.

Kirsten continues to recover in rehab, and the Cohen men continue to feel the affects of her absence. Sandy tries to convince Kirsten to come home, but Kirsten's attachment to Charlotte prevents her, which raises Sandy's suspicions. At Harbor, Dean Hess makes a decision affecting Marissa's future while Summer faces off against Taylor Townsend when planning the Kick-Off Carnival. Meanwhile, Jimmy makes Julie a surprising proposal.

Despite finally working their relationship out, Marissa and Ryan are forced apart by their parents. Dean Hess pressures Seth and Summer into volunteering, which leads them to come up with a way to give Ryan and Marissa a night to remember. Their plan is a success, but it lands Seth and Summer in hot water with Taylor. Meanwhile, the contents of Caleb's will are revealed, shocking both Jimmy and Julie.

Marissa adjusts to her new school and begins making friends at Newport Union, while Ryan worries that their separation will affect their relationship. Seth's anger with the Dean gets Summer in trouble, but when Summer uncovers shocking information about Taylor, they might have a way to bring the Dean down. Meanwhile, Sandy finds himself determining the future of the Newport Group and Julie finds herself in less than ideal surroundings.

Ryan makes a drastic, life-changing decision that Sandy, unlike everyone else, supports. Marissa, though not happy about his decision, turns to her new friends to help Ryan. As the Dean continues to try to ruin Ryan's future, Seth and Summer hatch a plan to expose the Dean's secret. Meanwhile, Charlotte makes Julie an offer she cannot refuse.

Taylor, after falling for Seth, prevents Summer and Seth from being together at a Harbor event by locking Summer out. Meanwhile, Marissa's friendship with Johnny begins to worry Summer, leading Summer to get Ryan involved. Meanwhile, at the Newport Group, Sandy works on a plan to save the company, and Charlotte convinces Julie and Kirsten to get back into the Newport social scene.

At The Subways' concert at The Bait Shop, Taylor's constant interference causes a rift between Summer and Seth. Marissa, worried about a confrontation between Ryan and Volchok, watches out for Ryan. Meanwhile, Matt, Sandy's new partner goes beyond what's expected and Julie manages to outdo Charlotte.

The seniors begin to prepare to apply for college and Summer, Seth, Ryan and Marissa begin to debate their futures. Marissa has a hard time deciding and ends up going to Johnny's surf competition instead of filling out applications. Meanwhile, Julie's financial situation forces her to return to her roots, and Sandy invites a friend from UC Berkeley to dinner with Seth and Ryan.

Ryan begins working as an intern for Sandy and Matt, and ends up working to save Matt instead. Determined to both get accepted to Brown, Summer and Seth do what they can to stand out. Johnny admits his feelings to Marissa, and Ryan and Marissa end up telling each other the truth.

In the spirit of Chrismukkah, the gang comes up with a plan to help Johnny -- a Chrismukkah Bar Mitz-vahkkah, involving Ryan getting Bar-Mitzvahkkahed. However, the holidays hit Summer hard when she begins missing her mother and Johnny decides to take his need into his own hands. Meanwhile, Kirsten, in the holiday spirit, reaches out to Julie.

Summer, Seth, and Ryan put together a plan to get Marissa back into Harbor. Johnny has to decide what's more important: Marissa's happiness or his feelings for her. Meanwhile, Julie Cooper and Dr. Roberts worry about telling their daughters the truth and Sandy has to make a moral decision that will change Marissa's future at Harbor.

Kaitlin Cooper returns to Newport from boarding school, bringing along her own share of secrets, attitude, and drama. She tries to reconnect with her family but is appalled by the trailer that her mother now lives in. Meanwhile, Kaitlin meets Johnny and makes sure that he knows she's available, especially since Marissa isn't.

Marissa puts all her effort into trying to get to know her sister better, but Kaitlin's attitude only causes trouble between Marissa and the rest of the gang. Meanwhile, Sandy tries to give Seth advice about college, but Seth becomes increasingly stressed about leaving Newport and Kaitlin offers Seth something to ease his nerves. Also, Sandy and Matt land a deal for a new hospital, Kaitlin and Johnny start hanging out, and Julie continues to try to put her family back together.

Summer finds out the truth about Seth's Brown interview, along with his other secret. Marissa admits her feelings to Johnny, but to cope with it, Johnny makes some bad decisions after drinking too much that endanger everyone around him. Meanwhile, Sandy and Matt hit a bump in their pending hospital project; Julie and Kirsten come up with a matchmaking plan to get Julie together with Neil Roberts, but when it backfires, Julie has to hide from him.

A tragic event takes its toll on the community, Kaitlin prepares to go back to school, Ryan questions his relationship with Marissa after lending a helping hand to Sadie, and Kirsten and Julie decide to go ahead with a Valentine's Day party.

In order to get in contact with Johnny's father, Sadie and Ryan go on a road trip together to find him. Despite Ryan regressing back to his old ways, the road trip brings Sadie and Ryan even closer, putting Ryan and Marissa's relationship at risk. Meanwhile, new evidence implicates Marissa in Johnny's passing and Seth and Summer find out the truth about what's going on between Julie and Neil.

After their falling out, Ryan debates whether or not he should invite Marissa to his 18th birthday party, while Sandy urges Ryan to invite one of his relatives. After months of putting it off, Neil and Julie finally tell the truth to Summer and Marissa, respectively. Meanwhile, Ryan and Sadie's relationship, as well as Marissa and Volchok's, continue to progress.

As Ryan and Sadie's budding relationship progresses, things start heading downhill when an old friend of Ryan's returns to Newport and Ryan decides to help her. Taylor tries to help Seth and Summer overcome their newest problem. Meanwhile, Marissa has to deal with being alone in Newport while Sandy deals with mounting business pressures.

After Julie and Neil announce their engagement, the Coopers move into the Roberts household. But Marissa starts to break apart from Summer and the others when she begins to spend more time with Volchok and his friends. As Ryan tries to progress his relationship with Sadie he gets stalled when Seth voices his opinions on the Ryan/Marissa situation. Meanwhile, Kirsten gives Seth a surprise and Sandy's work frustrations continue to grow.

When the gang receives their college acceptance and rejection letters, everyone must decide which path they want to take. Sandy and Kirsten become concerned about the decision that Ryan and Sadie come to. The letters lead Marissa and Summer work on patching up their friendship while Taylor decides to throw a College Sweatshirt Party to celebrate the letters arriving. Meanwhile, the toll that Sandy's job is taking on him begins to strain the Cohen household.

Ryan decides to visit his mother, but before he is able to do so, the advances of her co-worker delays him. In Newport, Taylor and Summer team up to figure out what Seth is hiding, but Summer is hurt in the process when their plan backfires. Meanwhile, Julie confronts Volchok in order to give him advice, and Marissa confronts Volchok's friends in order to get a girl out of a potentially bad situation. Sandy is able to fix the problems with Matt, but ends up neglecting Kirsten in the process.

As graduation rapidly approaches, Summer, Seth, Ryan and Marissa head to their Pre-orientations: Summer and Seth at Brown, and Ryan and Marissa at UC Berkeley. Kirsten is shocked to see a person from Ryan's past while dropping the boys off at the airport. Later, Seth runs into an old friend while at Brown, threatening his relationship with Summer. Meanwhile, Kirsten begins to relapse back into her old ways.

With Senior Prom in full swing, the good side, as well as the bad, comes out in everyone. Taylor hits an all-time high in her life, while Summer hits bottom with her prom date. Seth, with the help of Anna, devises a plan to win Summer back. Meanwhile, Volchok escorts Marissa to prom, and Ryan brings an unexpected girl from the past. Later, one of the girls is crowned Prom Queen, but the event is overshadowed when Ryan lapses back into his own ways when the Prom funds go missing.

After the fallout of Prom, Ryan and Volchok make a deal with each other. Sandy is chosen 'Man of the Year', but the district attorney digs deeper into Sandy's involvement in the building of the new hospital. Meanwhile, Marissa visits Kaitlin at her boarding school; and back in Newport, Theresa is forced to make a difficult decision and Seth's actions cause more trouble than good.

With senior year drawing to a close, graduation looms in the future, bringing the anticipation of college along with it. Seth, Ryan, Summer and Marissa together reflect on their past 3 years together and the major events that brought them all together. Seth has to deal with his parents and the police over his list of mistakes. Ryan's mother arrives in Newport with a surprise for him. Marissa is faced with a decision about her future after receiving a shocking proposal. Lastly, Summer has to face leaving Newport without Seth and has a hard time saying her goodbyes. Meanwhile, Sandy's big decision shocks Kirsten; Kaitlin returns home and moves in with Julie, wanting to be the next Cooper to attend Harbor; and Taylor is ecstatic when the 'core four' turns into the 'fab five'. But as everyone makes plans for their futures, their pasts might get in the way of their dreams.