Eduardo Fernando Balzaretti and his wife are a well-to-do couple spending part of the summer in a seaside house in Yucatan, along with their two unruly children: Mauricio Víctor César Villalobos and Sandro Alexis Castanares, who are nine and eight years old. The household is completed by the superstitious indigenous cook Yuriria Munguía and a local young fisherman Roberto Perdomo who serves as the children's underwater swimming instructor.
When the couple leaves for six weeks to cruise from Miami, the mother hires Mrs. Forbes Hanna Schygulla, a German governess, to look after the children. Mrs. Forbes arrives by boat after a long trip and is immediately instructed by the mother to impose "law and order" in the house and to treat the undisciplined and rude children severely in an attempt to improve their ways.
Once in charge, Mrs. Forbes adopts a formal, authoritarian, and repressive stance. She subjects the children to iron discipline, fueling a deafening resentment in them. Mauricio, the older boy, is more pliable, but Sandro takes longer to concede to her commands. From the start, the boys spy on her at night and discover that she drinks alcohol, eats sweets, and reads loud, strange passages in German.
Mrs. Forbes is also drawn to Aquiles Francisco Gattorno, the hunky young scuba instructor who spends a great deal of time around the house, both on land and at sea, often wearing very little. She attempts to seduce him, sharing tales that echo the myth of Achilles and Penthesilea, a love story framed as an impossible romance between legendary enemies. Although well put together, Mrs. Forbes is at least fifteen years his senior, and Aquiles remains repelled by her advances. After much insistence, Aquiles agrees to join her for a night out in town. In town, she pours out her true feelings, revealing that she despises order and restraint and secretly longs for freedom, drink, and a life of romance. Aquiles, however, is unmoved, and he guards a secret of his own—he is reluctant to let anyone near his seaside cabin, even the children who are close to him.
The following night, Mrs. Forbes dives into the ocean and resurfaces to swim to Aquiles's house. She enters secretly, only to find Aquiles naked in the company of a male lover. Before she can be discovered, she sets the house on fire with the two men inside.
By the next morning, over breakfast, Mrs. Forbes receives a letter that shakes her, but its contents are never revealed. Afterward, when she orders the day’s activities, the children have had enough of her control and disobey. They push back, and Mrs. Forbes finally relents, letting them do as they please. Their plan to regain freedom begins to take shape: after forcing the two boys to eat moray—the same fish they once feared when it was nailed to the door of the house—they hatch a more radical idea to poison her tequila. The plan is proposed by Sandro, the more resolute of the two boys, while Mauricio warns that it could lead to serious consequences, including the electric chair.
Yet the boys persevere. They poison the tequila and wake the next morning, thinking their scheme has succeeded. They celebrate their supposed liberty by scuba diving on their own. When they return, the beach house is in turmoil: Mrs. Forbes has been killed, stabbed many times. The brothers manage to slip past the police guarding the crime scene and enter the governess’s bedroom. There they find her lying dead, covered in blood with multiple stab wounds and cuts, rather than the poisoned tequila they had planned for. In death, Mrs. Forbes’s face shows an unsettling serenity, as if she finally found the peace she had claimed to crave.
