Daniel and Ana, a married couple from Madrid, arrive at a picturesque summer resort on the Costa Brava with their young son, Manuel, looking forward to a peaceful month-long vacation. However, their nine-year marriage becomes the target of a calculated psychological game when a local shopgirl, Marta, confesses her crush on Daniel to her cousin Robert. Intrigued, Robert orchestrates a cynical pact: he will help Marta seduce Daniel if she assists him in conquering Ana. As the cousins systematically manipulate scenarios to isolate the couple, the domestic boundaries collapse; Daniel succumbs to temptation and begins an explicit extramarital affair with Marta, while Ana finds her fidelity pushed to the absolute brink by Robert’s obsessive, calculating pursuit, turning their idyllic summer into a volatile minefield of emotional betrayal and psychological warfare.