Sara, an overweight teenage girl often ridiculed as Cerdita by her tormentors, Maca and her two friends, Roci and Claudia, navigates her life in a quaint town in Extremadura. During a fateful summer day, she decides to visit the local swimming pool, where she encounters her bullies who humiliate her by stealing her backpack and clothes. As she tries to return home, she becomes the target of harassment from a group of men and inadvertently escapes down a side road. Here, she spots a parked van that, unbeknownst to her, an unknown man uses to kidnap her bullies. Claudia, appearing through the van's rear window, pleads for help, but Sara turns a blind eye as the kidnapper, who had been present at the pool, leaves Claudia's towel behind before driving away. Choosing silence, Sara heads home and keeps the ordeal to herself.
The town is soon alerted to the mystery surrounding the pool, with the lifeguard found dead and the waitress missing along with the three girls. Accompanied by her mother Asun, Sara attends the pool’s inquiry, where both her mother and the local Civil Guard press her about her presence that day; she chooses to deny it. Later, Sara discovers she can track her stolen mobile phone, stolen by the very bullies who tormented her. Stealing her father's phone, she bravely ventures into the nearby forest and locates her backpack. In a chilling sequence, she encounters the kidnapper again, who warns her to stay silent while showing a bizarre form of affection before disappearing into the shadows.
Simultaneously, the families of the missing girls, including Elena, Claudia's mother, embark on their own investigation in the forest, which leads to the grim discovery of the waitress's lifeless body. After returning home, a friend of the missing girls, Pedro, visits Sara and offers her marijuana, revealing he knows she wasn’t truthful about being at the pool. He mentions the suspicion surrounding him regarding the disappearances, prompting Sara to pledge she will reveal what she knows to the Civil Guard should he be arrested. Their return to town spirals into chaos when Elena attacks Sara, and Pedro declares her secrets to the gathered crowd. Subsequently, Sara is taken in by the authorities for questioning, but her inability to continue due to her unexpected period interrupts the investigation.
Unbeknownst to both Sara and Asun, the kidnapper is plotting against them and successfully incapacitates Sara's father during a vicious attack. Following a heated argument between Sara and Asun, the kidnapper strikes again, subduing Asun before abducting Sara to an undisclosed warehouse. Here, she finds Roci and Claudia alive but restrained, desperately trying to save them. During her rescue efforts, she uncovers the horrific remains of Maca and is caught by the kidnapper, who attempts to manipulate her into killing her former bullies. Instead, in a fierce act of rebellion, Sara fights back, leading to a deadly chaos as the kidnapper accidentally injures Claudia. Fueled by adrenaline and survival instincts, Sara bites into the kidnapper's neck, ultimately killing him.
Empowered, she frees Claudia and Roci from their restraints by shooting the chains instead of them. With a newfound sense of freedom, she embarks on a journey back to town and unexpectedly crosses paths with Pedro on his motorbike, riding together towards a glimmer of hope.
