THE BEAST MUST DIE

The film opens with a narrator stating that the film is a detective story where the viewer is the detective and tells the audience to watch for the "werewolf break". Near the climax, the "werewolf break", which is a 30-second pause where the audience is asked to guess the werewolf's identity based on clues from the film, briefly interrupts the film.

Millionaire Tom Newcliffe invites a group of people, along with his wife Caroline, to spend some time in his rural English mansion, where he reveals that one of them is a werewolf and therefore must be killed. The group is composed of disgraced diplomat Arthur Bennington; Jan and Davina Gilmore, a pianist and his ex-student, now his wife; Paul Foote, an artist recently released from prison; and Professor Lundgren, an archaeologist and a lycanthropy enthusiast.

They all stay in the mansion where they are submitted to various tests to detect whether they might be a werewolf. The entire house is under surveillance by CCTV cameras, as well as motion sensors in the grounds around the mansion set up and overseen by Tom's associate Pavel, who does not believe in werewolves.

The only way to determine the identity of the werewolf is for a certain combination of elements to occur all at once, including a full moon and the presence of wolfsbane pollen in the air. When this fails to produce any lycanthropic reactions, Tom makes each of the potential werewolves grab silver objects to provoke allergic reactions, but this too proves unsuccessful. Later that same night, Pavel is killed by the werewolf, which makes Tom even more obsessive in his hunt, to his wife's increasing annoyance. Tom gradually focuses his suspicions on Paul Foote, who was reportedly arrested after eating human flesh. Foote denies being the werewolf as the creature continues killing, with the helicopter pilot, diplomat Arthur Bennington, and Caroline's dog all falling victim.

Tom subjects the remaining group to one final test: placing a silver bullet in their mouth. As Caroline submits to the test, her hairy, clawed hand is shown before she immediately transforms into the werewolf. She (fully transformed) attacks Tom, and he kills her by shooting her with a silver bullet, leaving him very distraught and confused because Caroline was alongside him when the werewolf killed her dog. Prof. Lundgren deduces that Caroline must have contracted the werewolf disease while taking care of her dog's wounds due to an open cut on her hand she sustained from a broken wineglass at dinner. Tom becomes enraged, convinced that Foote is the werewolf. When he attempts to confront him, however, he finds that Foote has also been killed. To avenge his wife, he enters the woods surrounding the mansion to hunt the werewolf. He finds the beast and finally shoots and kills it. Once dead, the werewolf reverts to its human form, and it is revealed to be Jan, the pianist.

Tom returns to Prof. Lundgren and Davina, and he realizes that he was bitten by the werewolf during the scuffle, thus condemning him to inherit the creature's curse. Not wanting to be a monster, Tom locks himself in the mansion and shoots himself in the head with a silver bullet, ending the werewolf's bloodline.

Tom discovers that his wife Caroline is the werewolf and kills her with a silver bullet; later, he hunts and kills the true werewolf, Jan, but is bitten in the process.

The ending delivers a double twist: Tom’s wife Caroline is revealed as a werewolf through a failed test, only for the real threat to be Jan, the pianist, who attacked Tom and is ultimately killed. The narrative underlines the futility and obsessiveness of Tom’s hunt, as his vigilance leads to personal tragedy and his own infection. The film closes the loop on its detective premise, yet leaves the consequences of Tom’s actions and fate hanging over the story.