Tormented by an unseen presence, a blind woman Sara, Belén Rueda prepares to hang herself in her basement, but changes her mind. As she tries to remove the noose, the stool beneath her is kicked away, leaving her to die. Miles away, Sara's twin sister Julia, Belén Rueda collapses, sensing something amiss.
Julia, Belén Rueda who shares the same degenerative disease but can still see, is tormented by Sara's death and by the feeling of another presence nearby. She insists Sara was not depressed as she was awaiting surgery to restore her vision. Her husband, Isaac, Lluís Homar, insists she stop investigating, as his doctor has informed him that Sara had eye surgery that was not successful.
Julia meets Sara's elderly blind neighbor Soledad, Julia Gutiérrez Caba, who has a pessimistic view that Isaac will leave her, as her own son Ángel, Pablo Derqui abandoned her after she went blind. After hearing that Sara had a boyfriend and they visited a hotel nearby, Julia goes there with Isaac. She is approached by an elderly janitor, Créspulo, Joan Dalmau, who warns her of "men who live in shadows," who are dangerous because they are tired of being ignored.
Isaac disappears and Julia is convinced that the invisible man has kidnapped him, though the police are skeptical. Someone kills Créspulo, but police rule it an accident. Julia's eyesight continues to deteriorate. When Julia and the inspector return to Sara's house, the inspector discovers a suicide note and Isaac's body. Julia, now fully blind, cannot see anything.
A grieving Julia learns Isaac's suicide note declared he loved Sara, with whom he had been having an affair for six months. However, an eye donor is found, so the operation to save Julia's sight goes ahead. She is told she must wear bandages to protect her eyes from light for two weeks, and the morgue agrees to keep Isaac's body so she can see him to say goodbye. She returns to Sara's house with the daily help of a home nursing aid, Iván, Dani Codina.
Julia is plagued by disorientation and convinced that somebody is lurking in the house; but Iván's patience helps her regain her independence.
Four days before Julia is due to remove her bandages, an unseen man almost succeeds in drugging her while she sleeps; however, she wakes, panicked, and accidentally hits the intruder. She flees to a neighbour, Blasco, who makes advances on her. She escapes, calling Iván, who finds her hiding outside in the rain and escorts her to his apartment.
While Iván is out, Julia hears the voice of Blasco's shy daughter Lía, Andrea Hermosa, who tells her that Iván is the invisible man who tormented and blinded Sara by ruining her operation, forced Isaac to write the fake suicide note before killing him, and has walls covered with photographs of Julia and Sara. Julia hides in the bathroom where, four days early, she tears off her bandages, desperate to see. Julia exits the bathroom and sees Iván's walls covered with photographs of the twins as well as Lía's bloodied body.
Iván returns to the apartment, where Julia pretends that she is crying because her operation has failed. He believes her momentarily before leading her to the body of the real Iván; Julia's scream betrays the fact that she can see. "Iván" tells her he loves her and wants them to be together as long as her sight is gone, since blind women are the only ones who need him.
Iván drives Julia back to Sara's, where she flees to Soledad's. Soledad is not as blind as she seems, and Angé is revealed to be her missing son, Ángel, Pablo Derqui. A candle and glasses betray the fact that Soledad is not really blind as she knocks Julia out, and Ángel blinds her. Julia escapes and manages to contact the police. She uses a flashlight to show the police a bloodied Ángel hiding in a corner of the room. Finally visible, he slits his own throat.
At the hospital, Julia is told the damage to her new eyes is irreversible. Using her last few hours of vision, she finally says goodbye to Isaac's corpse, as it is revealed that he donated his eyes to her.
