Pablo Quintero is a successful gay film and theatrical director whose latest work, The Paradigms of the Mussel, has just been released. At the opening night party, he discusses with his much younger lover, Juan Bermúdez, their summer plans: Pablo would stay in Madrid to work on a new project, while Juan would leave for his hometown in the south to work in a bar and stay with his family. Pablo is in love with Juan, but he realizes that his love is not returned with the intensity he desires.
Pablo is very close to his transgender sister Tina Quintero, a struggling actress who has recently been abandoned by her lesbian lover, a model, who left her in charge of her ten-year-old daughter Ada. Ada is precocious and perceptive, and she doesn’t miss her cold mother. She is happier living with Tina and spending time with Pablo, on whom she has a crush. Tina, Ada, and Pablo form an unusual family unit, and Pablo looks after them both. For his next project, Pablo writes an adaptation of Cocteau's monologue-play The Human Voice, to be performed by his sister.
At the play's opening night, Pablo meets Antonio Benítez, a young man who has been obsessed with the director since he watched The Paradigms of the Mussel. They go home together and have sex. For Antonio this is his first homosexual experience, while Pablo considers it just a lustful episode. Pablo remains in love with Juan Bermúdez, but Antonio misreads the situation and takes their encounter as a relationship.
Antonio comes across a love letter addressed to Pablo, signed by Juan, but which in fact was written by Pablo to himself. The letter makes Antonio fall into jealousy, but he has to return to his native Andalusia, where he lives with his domineering German mother. As he promised, Pablo sends a letter signed Laura P, the name of a character inspired by his sister in a script he is writing, telling Antonio that he loves Juan and intends to join him. However, Antonio, who is possessive and wants to get rid of Juan, tries to have sex with Juan. When Juan rebukes his advances, Antonio throws him off a cliff. After killing his rival, Antonio quickly heads for his hometown.
Pablo drives down to see the dead lover, realizes that Antonio is responsible for the murder, and confronts him about it. They have an argument and Pablo drives off, pursued by the police. Blinded by tears, he crashes his car and wakes in a hospital suffering from amnesia. Antonio's mother shows the police the letters her son received, the mysterious Laura P becoming the prime suspect, but the police cannot find her. Antonio returns to Madrid and, in order to get closer to Pablo who is still in the hospital, seduces Tina who believes his love to be genuine.
To help her brother recover his memory, Tina tells him about their past. Born as a boy, in her adolescence she began an affair with their father. She ran away with him and had a sex change operation to please him, but he left her for another woman. When that incestuous relationship ended, Tina returned to Madrid, coinciding with the death of their mother, and got reunited with Pablo. Tina has been grateful to Pablo who did not judge her. Tina also tells him that she has found a lover. Pablo gradually begins to recover; he realizes Tina's new lover is Antonio and that Tina is in danger. He goes with the police to Tina's apartment where she is being held hostage by Antonio. Antonio threatens a bloodbath unless he can have an hour alone with Pablo. Pablo agrees and joins him. They make love and Antonio then commits suicide.
