Rebecca, a young woman who was blinded in an accident caused by her boyfriend John imbibing drugs and alcohol. She was contacted by an estate agent, Marku, who informed Rebecca that she has inherited a castle in Albania from her mother Lavinia Whateley, who gave her up for adoption. Rebecca knows nothing of the woman, particularly that Lavinia had another child that she kept chained in the castle basement and who was freed by an unknown person. John and Rebecca's relationship has become strained after the accident and John's eagerness to sell the castle clashes with her desire to learn about Lavinia.Once at the castle Rebecca hears strange sounds and visions, some of which are caused by Lavinia's other offspring. John refuses to believe anything and instead continues to push for the sale of the castle and its furnishings. Things grow more tense between them after four of John's friends arrive, particularly as one of them is a woman John had been flirtatious with in the past, Shelly. Rebecca is pleased, however, to discover that one of the four, The Professor, is willing to listen to her and is also knowledgeable about the occult, having studied at Miskatonic University. The two investigate a Necronomicon Rebecca discovered and find a series of tunnels that go throughout the castle. Exploring the tunnels reveals Lavinia's journal, through which they discover that Lavinia was forcibly impregnated by her father during a ritual to Yog-Sothoth and that she gave birth to twin girls. Rebecca was put up for adoption while the other girl was born deformed and kept captive. Lavinia warns that once the two reunite and join hands, the gate to Yog-Sothoth will be opened. The Professor and Rebecca choose to leave the tunnels but are separated, during which time The Professor comes across Rebecca's sibling. He entreats her to show him "everything", as he wants to learn about the cult and the Elder Gods. She does, in doing so fully converts him into a believer.Meanwhile, John chooses to have blindfolded sex with Shelly, during which he is unaware that the monstrous sibling has killed Shelly and taken her place for the act. He's discovered afterwards by his remaining two friends and the three of them decide to hunt the murderer. This is unsuccessful, as she slaughters the friends, leaving John alive. Desperate to find Rebecca, John discovers that the monster has kept Marku captive and frees him despite having since learned that he lied about being an estate agent. This proves to be an unwise choice, as Marku betrays John and reveals that the whole goal was to bring the two siblings together and open the gate. John kills Marku and escapes to the outside, where he is attacked.Rebecca overhears the attack and discovers that he is prepared to kill her sibling. She begs for John to spare her sister, only for John to instead turn on Rebecca and begin to beat her. John is then killed by the sibling. Overwhelmed, Rebecca allows her sister to lead her to the ritual site, where The Professor urges her to embrace her destiny. She obediently takes her sister's hand as the gate opens to allow Yog-Sothoth into the world and Rebecca's body undergoes a monstrous transformation.In an after credits sequence The Professor enters his professor's office at the university, where he greets a man he calls "West" as a glowing vial sits on the desk.(copied from Wikipedia)