Popular and rebellious teenager Landon Carter (Shane West) is threatened with expulsion from school after he and his friends leave evidence of underage drinking on the school grounds and seriously injure another student as the result of a prank gone wrong. The head of the school gives Landon the choice of being expelled or atoning for his actions by tutoring fellow students and participating in the school play. During these functions, Landon notices Jamie Sullivan (Mandy Moore), a girl he has known since kindergarten and who has attended many of the same classes as him, and who is also the local minister's daughter. Since he's one of the in-crowd, he has seldom paid any attention to Jamie, who wears modest dresses all the time and owns only one sweater. Jamie is labeled an outsider and a geek. She makes no attempt to wear make-up or otherwise improve her looks or attract attention to herself.

Landon has trouble learning his lines for the play. Jamie, who is also in the play, agrees to help him on one condition: Jamie warns Landon not to fall in love with her. Landon and Jamie begin practicing together at her house after school. They get to know each other and a spark of affection arises between them.

On the opening night of the play, Jamie astounds Landon and the entire audience with her beauty and her voice. Onstage at the peak of the ending to the play, Jamie sings. When Jamie finishes, Landon kisses her which is not part of the play.

Jamie avoids Landon after the play, and it is not until Landon's friends play a cruel prank on Jamie and he protects her in opposition to his friends that she warms up to him again. Landon asks Jamie on a date soon after, but Jamie says her father doesn't allow her to date. Landon asks her father if he can date his daughter, bringing up that he's looking for a chance at redemption with her and at life through her. Reluctant at first, he gives in.

On their first date, Landon helps Jamie to fulfill her list of things she wants to achieve in life, such as being in two places at once, and getting a tattoo. After that, they go to the docks. Jamie tells Landon about how she experiences belief and how it's like the wind. It is then that he tells her he might want to kiss her now.

On another date, where Jamie is very silent and unfocused, Landon asks Jamie what her plans for the future are. She then confesses she isn't making any because she has leukemia and hasn't been responding to treatment. A desperate Landon asks for his father's help in curing her, but is disappointed by his reply and heads on a long drive home thinking about Jamie.

One by one, his friends become aware of the tragedy looming for Jamie and Landon. They give their support to him. Jamie's condition grows worse and she gets sent to the hospital.

Still in hospital, Jamie gives Landon a book that once belonged to her mother. She states that maybe God sent Landon to her to help her through the rough times and that Landon is her angel.

Unbeknownst to Landon, Jamie is given private home care by Landon's estranged father relieving her father's financial burden. Landon visits his dad, tearfully thanking him for his help. They embrace and are reunited.

Landon is building a telescope for Jamie to be able to see a one-time comet in the springtime. Jamie's father helps him get it finished in time. The telescope is brought to her on the balcony. She gets a beautiful view of the comet through the new telescope. It is then that Landon asks her to marry him. Jamie tearfully accepts, and they get married in the church in which her deceased mother got married. Jamie and Landon spend their last summer together, filled with a deep love like no other. Jamie dies when summer ends.

Four years later, Landon has finished college and been accepted into medical school. Landon visits Reverend Sullivan to return to him Jamie’s precious book that belonged to her mother. Landon apologizes to the Reverend that Jamie did not witness a miracle (an ambition she expressed in the class yearbook). The Reverend disagrees saying that in fact she did and that her miracle was Landon. He is shown to have completely changed his original opinion of Landon in the beginning of the film, where he completely detested Landon and did not hide it.

Landon visits the docks contemplating the belief that although Jamie is dead, that she is with him. It is then that he understands love is like the wind; you can't see it, but you can feel it.