Spain, 1950's. Montse (Macarena Gómez) has lost her youth taking care of younger sister Nia (Nadia de Santiago), both locked in a dark apartment in the center of Madrid. Their mother died during Nia's birth and their father (Luis Tosar) ran away, unable to handle the situation. And so, forced to act as father, mother and older sister, Montse hides from reality, feeding an obsessive, unhinged temperament. She suffers from agoraphobia and her only link to reality is Nia. That link breaks when Carlos (Hugo Silva), a neighbor of them, falls down the stairs and looks for help knocking on the only door he can drag himself towards. Someone has entered the shrew's nest, and might not come out again. In the end we learn that Montse is actually Nia's mother and older sister, for her father raped her after the mother died. After having Nia and raising her as a little sister, she kills her father when he starts to show sexual interest in 5-year-old Nia, and hides his body in the blocked fireplace. This is all learnt by the latter in the final moments of the film, the last scene depicting Nia choosing to hide, forever, in the "shrew's nest".