A bus transports five young couples to an ancient castle situated in an undisclosed location. They have answered an advertisement seeking participants for a 30-day medical study. The organization conducting the experiment, headed by the coordinator, informs them that there will be no communication with the outside world. Participants are free to leave whenever they wish, but doing so would mean losing their compensation of £20,000 per couple. As mysterious and inexplicable occurrences unfold around the castle, accompanied by eerie noises during the night, the group initially suspects that the company is fabricating a “haunting” to observe their reactions. However, they soon discover that something far more ominous is at play.