The Last Song revolves around seventeen year old Ronnie Miller, who is sent by her mother to spend the summer after graduation with her ten year old brother, Jonah, visiting her father, Steve. Steve, a former Juilliard professor, had divorced Ronnie’s mother, Kim, three years earlier and left New York City for his childhood home, Wrightsville Beach, North Carolina. Ever since, Ronnie has refused to talk to him or to play their shared passion, the piano, and becomes a rebellious clubber.
Upon arrival, Jonah immediately bonds with Steve over a stained glass window that Steve has been recreating for the local church after the original burned down in a mysterious arson. Ronnie, however, is hostile. At her request, Steve boards up his beloved piano and instead pursues his search for God’s presence. He also begins writing a piano composition using a piano owned by the church, but struggles with its ending. At the town’s annual carnival, Ronnie collides with beach volleyball player Will Blakelee as he dives for a ball.
Ronnie storms off and meets gothic Blaze, who befriends Ronnie and introduces her to Marcus, Blaze’s emotionally abusive boyfriend, and his cronies, Lance and Teddy. The three thugs earn money by putting on fire juggling performances for beach crowds. Blaze is crushed when Marcus becomes interested in Ronnie, and retaliates by framing Ronnie for shoplifting and violating a probation agreement. Marcus offers to make Blaze confess if Ronnie has sex with him, but Ronnie refuses.
Ronnie later confronts Blaze, and although Blaze admits she was wrong, she is trapped: she has been kicked out by her mother and is staying with Marcus, who forbids her to confess. Steve surprises Ronnie by believing her when she tells him she was framed, and she begins to soften towards him. Steve shows Ronnie, a vegetarian and animal lover, a Loggerhead Sea Turtle nest Jonah found near their home. Ronnie vows to protect the endangered creatures, bringing her back in contact with Will, an aquarium volunteer. Soon, Ronnie and Will fall in love.
They spend several blissful weeks together, though their romance is troubled; Will’s mother and ex-girlfriend attempt to drive them apart, and Scott, Will’s best friend and beach volleyball partner, is annoyed that Will is spending his time with Ronnie instead of practicing for their upcoming tournament. Marcus continues antagonizing Ronnie and blackmails Will with his knowledge of a secret: Scott was responsible for the church arson. Will and Ronnie also worry about the looming end of summer, which brings with it Will’s leave for Vanderbilt University and Ronnie’s shoplifting trial.
Ronnie attends to Will’s sister’s wedding. While dancing with Will at the party, they were accosted by Marcus, who goads Will into a fight which wrecks the wedding reception. Worried that the rich and powerful Blakelees will press charges, Marcus decides to raise money to leave town by putting on the most dangerous fire-juggling act yet the next day, the same day as Will’s volleyball tournament. The act begins properly, but Blaze misses a catch and her shirt catches fire. Her screams and the flames attract the attention of the volleyball tournament crowd, including Ronnie and Will.
As they drive Blaze to the hospital, Marcus and his crew steal away. Blaze recovers and reconciles with her mother. Jonah and Ronnie notice Steve is becoming progressively weaker. They are shocked when he begins to cough up blood the same night the Loggerhead nest hatches. Steve reveals the reason he asked them to visit: he is dying of stomach cancer, with only a few months left to live. Ronnie is further stressed when Will reveals Scott’s secret, causing her to furiously break up with Will two days before he leaves for Vanderbilt.
However, she reconciles with Blaze; Blaze confesses to framing Ronnie, but is released as an informant when she reveals the church arsonist was Marcus, rather than Scott. At the end of summer, Kim and Steve insist Jonah returns to school, but Ronnie refuses to leave. When Kim goes back to New York, Ronnie asks, her mom to send the letters that her father had wrote her, that she never had a chance to read. Months pass, and Steve’s health deteriorates. With no word from Will, Ronnie determines their time together is over.
Ronnie grows closer to Steve and secretly finishes his composition for him. In Steve’s final days, Ronnie breaks down the boarding around Steve’s piano and uses it to play the composition. Following Steve’s funeral, Ronnie moves back to New York, auditions for Juilliard, and is accepted. Ronnie is amazed to receive a visit from Will in New York, who says he didn’t call because Steve needed her more. He also reveals he has transferred to Columbia, which, like Juilliard, is in New York.