Five young couples travel to a remote castle for a 30-day medical trial. The company overseeing the experiment informs them there will be no outside contact, but they are free to leave early, forfeiting their £20,000 fee per couple if they do. As they settle in, mysterious and unexplained events begin to occur around the castle, accompanied by strange noises at night. Initially dismissing them as staged to test their reactions, the couples soon discover a more sinister truth behind the unsettling occurrences.