Note - almost everyone dies off screen.
It turns out the mother has split personalities and one is homicidal. She is the one that killed her husband, three children and family dog.
Cristian (Cristian Valencia) and July (Clara Moraleda) are two ghost-loving, aspiring videographers in their teens. They are going on vacation with their parents, Santiago and Debora (Chus Pereiro), younger brother Jose, and the family dog. They will be staying at their mother's childhood home in the countryside. Next to the home are some woods that are much like a labyrinth. Local legend has it that a young girl named Melinda, dressed in red, was lost and died in the woods, and her spirit still roams there at night. While many say that the spirit simply helps lost travelers find their way out of the woods, others claim the girl is malevolent. Despite both parents warning the teens to stay out of the woods, they sneak in on many occasions (marking their path with chalk) and find many things, including a well where Melinda is rumored to have died, and an altar. The teens also investigate the basement, finding many of Debora's childhood toys, diaries, a broken TV, VCR, and videotapes.
The dad has to leave the house for two days and strange things begin to happen. Weird noises emanate from the woods, and Cristian claims to see a figure while he wanders through the woods. One night, the family dog disappears. Jose is especially upset since he played more with the dog than his own siblings. Debora is also sick, lying in bed through most of the morning. Cristian and July decide to investigate the woods and they soon find a trail of blood. They follow the trail to the well and find the dog dead at the bottom of it, clearly killed by someone or something. The teens decide not to tell Jose or Debora about their discovery until the next morning.
That night, as Cristian and July are up in their rooms, Debora bursts through the door in a panic, asking them if they have seen Jose. When they say no, Debora flees downstairs and runs into the woods screaming out for Jose. Cristian and July, after grabbing their cameras, quickly run after her, but all become separated from each other (and July's camera breaks). As Cristian wanders alone through the woods, someone/thing grabs him from behind and drags him until he manages to escape and flee. He then finds his mother's flashlight lying broken on the ground. He finally wanders to the altar where he sees July tied to it. He frees her and they follow Cristian's markings back to the house.
While back in the house, July opens the fireplace grate to find Jose's charred body inside. The electricity then goes out. Cristian helps July hide in the pantry, but before he can find his own hiding space, someone begins chopping through the front door with an axe. Cristian runs upstairs and barricades himself in his bedroom. When he goes downstairs in the morning, he finds the entire house in complete disarray. July is no longer in the pantry, which is now covered in blood. Cristian hears noises from the basement and goes downstairs, to find someone has managed to set up the TV and is playing a tape on the VCR.
The film then cuts to police photos of the crime scene and plays a call Santiago made to the police when he returns home. The dad says that everyone is dead and he is in the house. We then hear Santiago scream and are informed that the police found four dead bodies when they arrived - Santiago and the three kids.
The film jumps back to Cristian in the basement. He begins to watch the tape, an interview with his mother, Debora. She looks sick and the doctors say that Debora becomes violent, especially at night. They believe it has to do with some childhood trauma. The doctors also note that she has already killed one of her infant children during the night, saying her psychosis is exacerbated by children. They then ask her about another personality she has called "Elvira." As Cristian watches the tape, his mother creeps up behind and kills him with an axe. Her whereabouts remain unknown (and the Melinda legend was a red herring to throw off the audience).
Thanks Evan B!