In the fourth season premiere, the residents of Tree Hill must deal with the aftermath of the shocking accident that left everyone without words. In the midst of all the chaos, Dan extends his help to Karen now that she is pregnant, Lucas and Peyton's friendship continues to grow stronger, and once rivals Brooke and Rachel move past their differences and manage to become good friends. Brooke has a difficult decision to make about her relationship with Lucas and Skills decides to try out for the Ravens.
It's her eighteenth birthday, but Brooke soon realizes that she has no one but Rachel to celebrate it with. Meanwhile, Nathan continues to struggle with memories of the accident and Dan continues to be haunted by remorse over his actions against Keith.
Peyton spends more time getting to know her newfound brother, Derek. Meanwhile, Lucas gives up on his love for Brooke and Rachel tries to start a romance with Nathan.
Rumor surfaces that Brooke might be pregnant, causing Karen to worry as she fears that the baby might be Lucas'. Meanwhile, Rachel continues to get closer to Nathan when she confesses that she saw Keith during the accident.
Nathan gets two surprises in a row, and everyone is eager to share their advice with him; Deb also shares something she picked up; and Brooke decides to celebrate. Peyton's stalker (her brother) attacks her and her real brother saves her along with Lucas.
Nathan struggles with money problems and reluctantly asks Dan for help; Brooke learns the new man in her life is also her new English teacher; Peyton gets help from an unexpected source; and Lucas returns to the Ravens just in time to help the team advance in the playoffs for the state championship.
Peyton returns to school, but she breaks down; Brooke's fashions are a hit at the local show; Nathan is cornered into a no-win situation by Daunte; Deb takes drastic action for her addiction.
Derek tells Peyton to give Lucas a chance just as Brooke tries to make up with him; Nathan has more trouble with Daunte; Rachel asks Haley to tutor her when she discovers she is failing a class.
As the Ravens prepare for the championship, Lucas learns Nathan intends to throw the game and decides on a risky plan to even the odds; Karen insists Deb show up for her son's final game and tries to help her get there; Dan makes a last plea with Daunte.
After a cardiac incident, Lucas has a dream in which the spirit of Keith visits and shows him how much one life can affect the lives of others, with a nod to It's a Wonderful Life. Elsewhere, Haley is the victim of an accident and Dan helps out Nathan.
The aftermath of the series of unfortunate events at Tree Hill begins. Lucas returns to his everyday life, following his heart attack, and tries to rekindle his relationship with Peyton. However, things are rougher for Haley when she discovers that Nathan had more involvement in her accident than she could have ever imagined.
Haley finds herself frustrated over her injuries while Skills and Mouth help Nathan find a job as a stripper in order to earn money for the senior prom. Meanwhile, Brooke betrays Rachel in order to get with a new guy and Lucas and Peyton finally reach happiness in their relationship.
A class assignment reveals the deepest secrets and desires of Lucas and the students at Tree Hill High. Worried about college prospects, Skills turns to Haley, while Nathan wrestles with Deb's suicide attempt. Chase learns that Brooke cheated on her calculus exam, while nudity and drugs run rampant in the halls of the school.
Haley confronts Brooke regarding the stolen calculus test. Lucas and Peyton consider taking their relationship to the next level as Peyton and Brooke grow closer. In Deb's absence, Nathan throws a senior party at the Scott House where a sex tape from Nathan's past surfaces.
It's Prom day, and emotions are flying high at Tree Hill. An emotional Haley confronts Nathan about the sex tape and asks for a list of all his past sexual encounters. Meanwhile, an angry Peyton asks Lucas to skip the dance altogether, but he refuses to take no for an answer. Finally, Brooke and Peyton let out all their tension in a confrontation that can tear them apart forever.
It's Prom night, but things start to go awry as Lucas discovers who was the person in the hallway when Keith was killed, and Psycho Derek holds Peyton hostage at her own house. Meanwhile, Haley and Nathan vow to be romantic with each other throughout the night, and Dan continues to win over Karen's affections.
When Mouth and Rachel's road trip goes wrong, the other teens go out to their rescue. While on the trip, Nathan and Haley get a second chance to enjoy their Prom and Peyton and Lucas decide to take their relationship to the next level. Finally, Brooke reveals the truth about the stolen calculus exam to Haley.
Brooke and Peyton finally rekindle their friendship, Nathan's past mistakes continue to haunt him and Haley, and Lucas hears a shocking confession from the witness to Keith's hug.
As the end of the school year approaches, Peyton considers participating in a music internship in Los Angeles, while Brooke tries to reconnect with Chase through yearbook signatures, and Haley struggles to write her valedictorian speech. Meanwhile, Dan suggests that Lucas get professional help when Karen worries about his obsession with theories about Keith's hug.
As graduation approaches, Haley prepares for the recital of her valedictorian speech and motherhood, Nathan must adjust to life without basketball, Brooke makes a confession that may have great consequences, and Lucas confronts Dan after Karen's life is threatened.
Dan must face the harsh consequences of his decision to hug Keith as Karen and Haley's pregnancies complicate the lives of Nathan and the graduating seniors of Tree Hill High. Lucas wrestles with his post-high school options, and Peyton and Brooke prepare to spend the summer in Los Angeles together.