It stars Belén Rueda as Laura, a woman who returns to the orphanage where she stayed for a period as a child. She purchases the house, with plans to turn it into a home for disabled children. Everything seems to be going well for Laura, her husband Carlos (Fernando Cayo) and their son Simón (Roger Príncep).
Laura, with her physician husband Carlos, returns to an orphanage where she had lived as a child with plans of reopening it as a home for sick and disabled children. They adopted a son named Simón who is HIV-positive, though he is unaware of either his adoption or his illness. Simón has imaginary friends and his mother is concerned that he is too old for them. He is scared in his large, dark new home and Laura tells him that the invisible light from the lighthouse at the beach will protect him.
Simón is reading the story of Peter Pan. He asks Laura about Peter Pan and why Wendy couldn't go back to Neverland when she grew up, and he makes the comment that he will not grow up either, just like Peter and like his imaginary friends.
A mysterious social worker appears one day, talking obliquely of some new treatment for Simón and revealing to Laura that she has his old adoption file. Laura becomes angry at her intrusion with these hidden truths and sends her away, locking Simón's file in a desk in the kitchen. That night, after investigating some rattling noises she hears outside, Laura finds the mysterious social worker hiding in a shed. Though Laura screams for Carlos' help, the woman escapes before anything else occurs. After they report the incident, it is discovered that there is no social worker registered with the name Benigna, which the woman gave.
One day, Simón tells Laura about a game that Tomás has created for him, a type of scavenger hunt. They find something in a place where it's not supposed to be, take it to its original place and find something else, and so on. Simón says that at the end they should find gold coins and that he will then get a wish. The game leads them to the locked drawer where Simón's file is hidden. Simón angrily reveals to Laura that he knows that she is not his real mother and that he is going to die. When she asks him how he knows that, he responds that his new friends told him.
Soon after, Laura hosts a party for the disabled children to welcome them to their new home. When she asks Simón to come down and join them, he first demands to show her where Tomás' "small home" is. After a brief argument, during which Laura angrily slaps Simón's face in the heat of anger, she leaves Simón alone upstairs. Back in the garden, she watches the children in play, all of them eerily wearing masks that Carlos is taking out of an old trunk. She and Carlos exchange looks as she nods upstairs, indicating that Simón is still in the house.When Laura returns to check on Simón, she cannot find him..
Well it's pointless to keep spoiling the movie to those who has not watched yet. Enough said! As long as Guillermo del Toro is in it, the movie is going to be a good one! I suggest watching in the dark...