Sara is an overweight teenage girl living in a small town in Extremadura. She is bullied by other teenage girls Maca and her friends Roci and Claudia, who call her Cerdita. During the summer, Sara visits the local swimming pool, only to be discovered by the bullies, who attack her and steal her backpack and clothes. Trudging home in her bathing suit, she is harassed by a group of men who chase her. She escapes onto a side road, where she sees a parked van which, initially unbeknownst to her, an unnamed man has used to kidnap her bullies. Claudia appears through the window, begging Sara to save her, but Sara is frozen with fear. Sara locks eyes with the kidnapper, who silently leaves a towel before driving off. Sara returns home, deciding not to tell anyone about what happened.
News reaches town that the pool's lifeguard has been found dead, and the waitress has gone missing, along with the three girls. Sara is questioned by both her mother Asun and the local Civil Guard. Sara denies that she was at the pool that afternoon. She realises she can track her mobile phone (which was in the backpack the girls had stolen) to determine the whereabouts of the girls. She steals her father's phone and ventures into the forest, where she locates her backpack and phone. Again she encounters the kidnapper, who warns her to be quiet and displays affection toward her before escaping. Family members of the missing girls, including Claudia's mother Elena, undertake their own investigation in the forest, during which they find the body of the missing waitress.
Pedro, a friend of the missing girls, introduces marijuana and reveals that he knows she was lying about not being at the pool. The townspeople suspect he is responsible for the girls' disappearance, and Sara promises to disclose what she saw if he is arrested. A hysterical Elena attacks Sara before Pedro reveals her truth to the crowd. The Civil Guard takes Sara for questioning. She succumbs to their pressure and begins to tell them information, but the sudden onset of her period prevents further interrogation.
Unbeknownst to Sara and Asun, the kidnapper has come to their home and attacked Sara's father. Sara and Asun have a heated argument, during which the kidnapper incapacitates Asun and takes Sara to an abandoned warehouse. There, Sara discovers Roci and Claudia, who are still alive but have been gagged and are hanging by their wrists. She attempts to untie them, but the kidnapper returns before she can complete the task. She discovers Maca's decomposing body and is found by the kidnapper, who embraces her. He encourages her to kill the two girls with his knife, but she attacks him instead. He accidentally fires his shotgun, blowing off Claudia's hand. Sara grapples with the kidnapper and kills him after biting a chunk out of his neck. She grabs the shotgun and contemplates killing her former bullies, but she shoots the ropes suspending them. She leaves them. As she walks along a road, she encounters Pedro on a motorbike and rides with him back to town.
