The story begins in a suburb of Mexico City with Ulises, whose father sells clothes in the market. He is accustomed to living humbly as he belongs to a low socioeconomic class, and could be considered a "naco". Ulises' environment is one of drugs, crime and graffiti, his great passion. One day, Ulises and his friends decide to go play Qzar, a laser-gun game. In the game, Ulises encounters Francisco, an upper-class boy, who is Renata's boyfriend. The "nacos" and the upper-class kids begin to fight after Francisco insults them. The fight ends in shoves and blows that only hurt Francisco's pride.

Ulises' life takes an unexpected turn when he meets Renata, an upper-class girl, in the Santa Fe Mall. Renata is buying clothes with her friend and confidante "La Güera" and younger, alcoholic sister, Mariana; when suddenly crosses seductive glances with Ulises through a shop window. Ulises feels strangely and deeply attracted to this girl and does not know why. Despite knowing the socioeconomic differences between them, he follows her while Renata and her companions try to flirt with him. Renata’s friends tell Renata to kiss Ulises and they share a passionate kiss. After that moment, Ulises continues to follow the girls in order to find out more about Renata.

Ulises encounters Francisco again while following Renata. Francisco realizes that Ulises has been following Renata and orders his bodyguards to catch him. They both recall their first encounter in Qzar, causing Francisco to assault Ulises. Renata tries to stop the fight but before she can, Ulises manages to get away and escapes from the mall.

Ulises cannot stop thinking about Renata and seeks her, knowing she always visits the mall. Ulises regains hope when he does, in fact, see Renata again. This time, she is with her mother, so the meeting becomes more difficult to realize. Ulises takes advantage of Renata’s mother’s carelessness to approach and talk to her, who is now more attracted to him. Renata gives her phone number to Ulises to arrange a future meeting.

A few days later, Ulises calls Renata at her house, a very large and elegant residence, telling her to turn on her radio. It is revealed that he had called a radio program and dedicated a song to her. He goes on to invite Renata on a date. Renata accepts, and "La Güera" becomes her accomplice, keeping the relationship a secret and providing an excuse for Renata's outings. The date takes place in the same place they met, Santa Fe. Multiple clandestine dates follow until they become an item. The uncomfortable unbalance between the social classes of Ulises and Renata unleashes disagreements between Mariana and her upper-class friends against Ulises' "naco" friends. Ulises and Renata are troubled by these events but decide to disregard them for the sake of continuing their relationship.

Renata is determined to bring a deeper relationship because Ulises thinks that she is a great girl. Ulises gifts range from cute portraits of her face, handmade by Ulises by himself, to huge painted graffiti on walls made especially for her. This makes Renata feel very special, because she had not previously had that kind of feelings being with boys like Francisco, who offered only trivial and material things. For a while, Ulises and Renata became inseparable, they even skip school in order to spend more time together. Francisco finds out and takes revenge on Ulises out of jealousy; he sends his bodyguards to beat him at Renata’s school where Ulises had been leaving her after dating. Renata cannot do anything to prevent the beating.

When Ulises shows his wounds and physical marks of the assault to his mother, she advises to him to stop seeing Renata. The reaction of Ulises’ friends is more radical as they decide to go to Renata’s school to get revenge. They start beating the rich kids in a massive and bloody confrontation. Many of the rich kids were injured, specially "La Güera"'s boyfriend, who ends up in the hospital, even though the vast majority were innocent. This reaches the ears of Renata’s parents, who had forbidden her to see Ulises again. She tries to defend their relationship, but their parents do not understand her. Renata is completely isolated from the outside world, so when she finally manages to send a message to Ulises, just to say goodbye, because she has decided that is the best for both. Ulises thinks that she does not love him anymore, but what Renata really wants is to protect him. Ulises does not take Renata’s decision well and begins to mourn and lament. He is on the brink of madness. His friends try to comfort him, but their efforts are futile as the only thing that could bring calm to him is to be next to Renata again.

Ulises is determined to see Renata again, despite the opposition from her parents, so he climbs over the roof of Renata’s house and goes to her room where their encounter culminates physically. After their meeting, they decide to get away together to Acapulco, where nothing and no one can prevent their love. Mariana overhears their plans and she calls Francisco to tell him. Before leaving, Ulises goes home and leaves his parents most of the money he earned since he began working. The couple goes to the bus station and just when Ulises and Renata are about to leave, Francisco and Mariana appear to prevent them from escaping. Desperate to keep Renata, Francisco pulls a gun and threatens to kill Ulises if they do not stop. Ulises is not scared as he wants to be with Renata so he challenges Francisco to do so, but unexpectedly Francisco, who is inexperienced in the use of arms, shoots Renata accidentally in the chest. A few moments later, Renata dies in the arms of Ulises, but not before telling him she loves him. Ulises goes into shock and refuses to accept her death, and the loss of the love of his life. The last scene shows Ulises mourning by the grave of Renata.

The drawings made by the character Ulises are really made by Luis Fernando Peña, the protagonists of the film, except for the graffiti, which was made by professionals. When filming the tape, the actors could not sleep at their own houses because they had to work for 24 hours.

The name of the movie is a game of words in Spanish: "Amarte" and "Amar te". The first one is the accusative second person singular, while the second one is the infinitive verb and the second person reflexive pronoun. Resuming, the first option would be literally translated as "To love you hurts" (It hurts to love you) and the second option as "To love hurts you" (It hurts you to love). It implies the pain Renata and Ulises feel by loving each other (amarte duele) and the pain that loving someone else provokes (amar te duele), like in the scene where Ulises' mother is curing him after the beating at Renata's school, so she suffers together with him.