Millionaire Tom Newcliffe invites a select group of individuals to enjoy a stay at his exquisite rural English mansion, accompanied by his wife, Caroline. As the gathering unfolds, Tom drops a shocking revelation: one among them is a werewolf, and for their safety, that individual must be terminated. The attendees consist of notable figures, including Arthur Bennington, a seasoned diplomat; the talented duo of Jan and Davina Gilmore, a pianist and his former student-turned-wife; and Paul Foote, an artist fresh out of prison. Additionally, the esteemed Prof. Lundgren, an archaeologist with a keen interest in lycanthropy, rounds out the eclectic group.
During their stay, the group undergoes a series of increasingly bizarre tests designed to draw out the werewolf. The mansion is under constant surveillance, outfitted with CCTV and motion sensors, all monitored by Tom's skeptical associate, Pavel, who scoffs at the notion of werewolves. Tom's strategy involves exposing the guests to silver objects to provoke reactions, but the initial attempts yield no results.
One fateful night, while patrolling the mansion grounds, Tom discovers a mysterious movement, only for Pavel to meet a grisly end at the hands of the very creature they fear. This incident drives Tom into a state of paranoia, particularly fixating on Paul Foote, who has a shadowy past involving cannibalism. Despite protests of innocence from Foote, Tom's unease grows.
In a desperate bid for answers, Tom even rents a helicopter for an aerial surveillance mission. While on this flight, he spots the beast racing through the woods and attempts to shoot it down, but his efforts are futile. The werewolf retaliates by killing the helicopter pilot and then attacks Caroline's beloved dog, marking its brutal return to the mansion where it claims Arthur Bennington in a shocking assault while he sleeps.
Throughout the film, viewers are invited to piece together clues about the werewolf's identity alongside the terrified group. A unique moment arises near the climax, dubbed "The Werewolf Break," where audiences are encouraged to speculate on the lurking monster's identity based on observed events.
As tension escalates among the remaining members, Tom devises a crucial final test: having a silver bullet placed in their mouths. Ironically, he must go first to prove his own innocence. However, the tables turn dramatically when, as Caroline takes her turn, a hairy, clawed hand is revealed, and she metamorphoses into the fierce creature. In a heartbreaking climax, Tom is forced to shoot and kill his wife with a silver bullet, unable to bear the thought of her transformation coming just after the werewolf had claimed her dog.
Meanwhile, Prof. Lundgren uncovers that Caroline had contracted the werewolf curse while tending to her pet's injuries. Misguided anger leads Tom to suspect Paul, only for that theory to crumble as Foote is discovered dead moments later, also at the hands of the werewolf.
In a dramatic act of vengeance, Tom ventures into the woods to confront the beast, ultimately shooting and killing it in a final showdown. To his disbelief, the werewolf reverts to its human form, revealing itself to be Jan, the very pianist Tom had initially trusted.
As the dust settles, Tom realizes the horror of his fate; he has been bitten during their scuffle, and the curse will soon be his. In a last-ditch effort to escape becoming what he fears most, Tom secludes himself in the mansion, retreating to his study where he tragically takes his own life with a silver bullet, marking a devastating end to his harrowing ordeal.
