The aftermath of the post-wedding car accident unfolds. While Haley is deeply affected by Nathan's attempt to rescue his uncle Cooper and Rachel Gattina from the underwater wreck, Brooke confronts Lucas with her true feelings, and Peyton makes a startling discovery. Also, pressured by the thought that someone knows he killed his brother, Dan assaults Deb.
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As the lives of Nathan, Rachel and Cooper remain turbulent following the car accident, Brooke celebrates her 18th birthday but finds it diminished by her struggles with Peyton. Lucas and Karen come to terms with her pregnancy and that of someone else close to Lucas. And Haley becomes aware of Deb's past addictions.
Lucas and Brooke, now broken up, clash at a post-game party, and Haley realizes that Rachel is plotting to seduce Nathan. Meanwhile, Peyton connects with her half-brother; Rachel reveals that she knows who is pregnant; and Skills takes the court with the Ravens basketball team.
Peyton introduces hip-hop sensation Lupe Fiasco at the nightclub Tric, while rumours that Brooke is pregnant run rampant. Still dealing with the car accident, Nathan buys a motorcycle, despite Haley's reservations. Meanwhile, Lucas clashes with Derek, a newcomer who is very interested in Peyton.
When Lucas discovers that newcomer Derek's interest in Peyton is more than brotherly, Derek becomes menacing. Haley feels Nathan slipping away after he receives a scholarship offer to attend Duke University. Meanwhile, as rumours of Brooke's pregnancy spread, Rachel uses the Internet to set Brooke up on a blind date with an older guy. And at Nathan's request, Deb tries to overcome her pill addiction.
Nathan approaches Dan for help with his and Haley's finances. Despite his heart condition, Lucas rejoins the Ravens, teaming with Skills. Feeling isolated and alone, Peyton receives comfort from an unlikely source. Brooke begins dating again but discovers that her new boyfriend is not all that he seems to be. Mouth gets an unexpected proposal. And an old acquaintance of Dan's, Daunte, shows a sudden interest in Nathan.
Brooke copes with the T.A.R.T. fashion show and a new boyfriend. Rachel makes the cover of Maxim magazine, causing chaos at Tree Hill High. Lucas learns of Nathan's dealings with Daunte, and ultimately has to choose between Nathan and his love for basketball. Haley takes a babysitting job but finds herself in over her head. And Peyton's half-brother, Derek, forces Peyton to confront her deepest fears.
Derek convinces Peyton to take a chance with Lucas - but, at the same time, Brooke is approaching Lucas about getting back together. Meanwhile, Daunte's latest demand leads Nathan to seek help from an unlikely source: Dan. When Rachel learns that she has gotten a failing grade, she turns to Haley for tutoring.
As Whitey's last game approaches, the residents of Tree Hill feel the pressure of a looming state championship showdown. Meanwhile, Peyton reveals to Lucas that she has feelings for him. Haley learns the sex of her baby. Nathan and Lucas clash over personal and team goals. And Dan tries to strike a bargain with Daunte.
Lucas receives an unlikely visitor: his supposedly dead uncle Keith. Meanwhile, the lives of Haley and her unborn child are threatened when she experiences serious complications with her pregnancy. Nathan wrestles with the guilt and consequences of his dealings with Daunte. Dan tries to make amends for Keith's murder. And Peyton and Brooke try to patch up their differences.
Following his heart attack, Lucas returns to school, hoping to rekindle his romance with Peyton. Haley learns that Nathan was responsible for her accident. Brooke and Rachel devise a plan to steal the next calculus exam. Dan faces murder charges. And Deb breaks out of her rehabilitation center.
With senior prom only weeks away, Haley's frustration over her injuries increases. Skills and Mouth try to help Nathan earn money for the big dance by stripping. Brooke betrays Rachel in order to get closer to a new guy. Peyton and Lucas resolve to be happy. And, struggling with her drug addiction, Deb makes a choice that could end her life.
A class assignment reveals the deepest secrets and desires of Lucas and the students of Tree Hill High. Worried about college prospects, Skills turns to Haley. Nathan wrestles with Deb's suicide attempt. And Brooke's new classmate, Chase, learns that Brooke cheated on her calculus exam.
Haley confronts Brooke regarding the stolen calculus test. Meanwhile, Lucas and Peyton consider taking their relationship to the next level. Peyton and Brooke grow closer. And, in Deb's absence, Nathan throws a senior party at the Scott house, where a sex tape from Nathan's past surfaces.
On the day of the prom, everyone in Tree Hill experiences the repercussions of Nathan's pre-prom party. Haley asks Nathan for a list of his past sexual conquests. Rachel is suspended after taking the blame for stealing a test with Brooke. Peyton tells Lucas that she wants to skip the prom entirely, but he refuses to take no for an answer. And Mouth and Brooke deal with their "Clean Teens" dates on prom night.
Lucas discovers who witnessed his uncle's killing a year ago. Meanwhile, Peyton's decision not to attend the prom leads to dire consequences. Nathan and Haley vow to just be romantic teenagers at prom. And Dan tries to win Karen's affections, as the witness to the murder he committed closes in.
In an episode shot on location in Honey Grove, Texas, as part of a national contest, the teens of Tree Hill mount a rescue mission to save Mouth from his ill-advised road trip with Rachel. During the journey, Haley and Nathan get a second chance to enjoy the prom, and Lucas and Peyton take their relationship to the next level. Meanwhile, Brooke tells Haley that she lied about stealing the calculus exam, and Nathan struggles with the residue of a past mistake.
Nathan fears for his future as past mistakes come back to haunt him and Haley. Lucas hears a shocking confession after making contact with the mysterious witness to the murder of his uncle. And Brooke rekindles her friendship with Peyton after crossing paths with Chase.
Nathan and Haley grow closer, despite the recent scandal. Brooke wonders if a spark might be rekindled with Chase. Peyton receives a tempting offer that could ultimately take her away from Tree Hill. Mouth reconnects with Gigi. And Lucas tells Karen that Dan is a murderer, setting off an explosive series of events.
As graduation day at Tree Hill finally arrives, Brooke's guilty conscience prompts her to make an important decision, while Haley prepares her valedictorian speech. Peyton accepts an internship in California. Karen experiences problems with her pregnancy. Unable to get a scholarship at any college due to his recent scandal, Nathan tries to define his future. And Lucas tells Deb the truth about what Dan did.
The Tree Hill High School graduates share their bittersweet final moments together. At the same time, both Karen and Haley go into labour, while their loved ones stand by. Brooke has a surprising reunion with someone from her past. Peyton struggles with the thought of spending the summer away from her friends. Dan tries to assuage his guilt by making a life-changing decision. And, reflecting on the amazing journey that has brought them to the point of being true brothers, Nathan and Lucas seek closure regarding Dan.