Clara, Isabél Zuaa, interviews for a nanny job despite having no experience or references. Just as she is about to be turned down for the job, the woman interviewing her, Ana Marjorie Estiano, experiences cramping. A sympathetic Clara guides her through the pain, and Ana decides to hire her as a live-in housekeeper and future nanny.
On her 29th birthday, Ana gets drunk and reveals to Clara that she is estranged from her family. Originally engaged to a man, she cheated on him and conceived a child from her affair. Her parents sent her to the city to have an abortion, but they severed ties with her after she refused to go through with it.
Ana is ordered by her doctors not to consume meat. One night, Clara finds her rifling through the fridge. When Clara tries to guide her to bed, Ana kisses her but then scratches her hard enough to draw blood. The following morning, Ana does not appear to remember the incident.
Clara realizes that the incident might be connected to the full moon. The following full moon, Ana passionately kisses Clara and the two have sex. Later that night, Ana sleepwalks again. Clara witnesses her murdering and eating a cat. The following day, Clara tells Ana about her sleepwalking. She also puts some of her blood into Ana's meal to relieve her thirst for blood and sleepwalking.
On the following full moon night, Ana's cramping worsens. Before Clara can call for help, Ana's stomach ruptures. She dies, and Clara sees that the surviving baby is a werewolf pup. Clara runs away and tries to abandon the baby, but changes her mind on the latter upon hearing the child's cries and decides to adopt him, naming him Joel as Ana wished Miguel Lobo.
Seven years later, Clara works as a nurse and has been fostering Joel as her own, raising him as a vegetarian and chaining him up in what is known as the “Little Bedroom" during full moons. After Clara's landlady, Dona Amélia Cida Moreira, gives Joel meat one day, he searches through Clara's things, where he uncovers a photo of Ana, becoming hostile with Clara and even transforming during an argument with her. The next morning, Clara reluctantly admits that Ana is his birth mother, but insists that she does not know who his father is.
Joel decides to search for his father, the only clue being a receipt from a shopping mall where Ana bought shoes. He and his friend Maurício Felipe Kenji go to the mall, but they are locked in when they decide to stay inside past closing time. Joel transforms under a full moon and soon after pursues Maurício to eat him whole.
Joel returns home alone before unconsciously transfiguring back, but as he retains excess hair and claws, Amélia discovers his secret when finding him with Clara and wants to perform an exorcism on him. Clara injects the landlady with an anesthetic and then plans to run away with Joel the next day, although he does not fully remember his actions in wolf form.
After locking her in the Little Bedroom in a fit of confusion and anger, Joel tries to attend a dance with his classmate Amanda later that night. There, he quickly realizes his true form and begs Amanda to leave before he once again turns. Clara, having managed to escape with help from a co-worker, shoots him in the leg before he can kill Amanda, non-fatally injuring him.
Clara takes Joel home to the Little Bedroom and removes the bullet, healing the wound. Meanwhile, a petrified Amanda tells the locals what happened, leading them to hunt Joel down, tracking down his and Clara's apartment. Amélia wakes up and watches a rioting mob of the locals with Amanda, both with implicit regret. With a lullaby Ana sang earlier in the film, Clara manages to tame Joel so that he does not attack her. She finally decides that she cannot keep him locked up and hungry. As the mob bangs on the door, Clara and Joel prepare to face them together as Joel lets out one final howl.
