In 1934, on the twentieth anniversary of his death, the remaining relatives (all cousins) of Cyrus West are summoned to his mansion in rural England to view his filmed reading of his last
will and testament. Among them are former surgeon Harry Blythe; flamboyant stuntman Charlie Wilder, who has forged a successful career in
California; Susan Sillsby, a noted English hunter; Cicily Young, Susan's female lover, also her cousin and roommate; the marginally successful American songwriter Paul Jones; and Annabelle West, a rising fashion designer. Overseeing the gathering is West's lawyer, Allison Crosby, and Mrs. Pleasant, his longtime maid who looks after the now-abandoned manor. Cyrus lets it be known in his reading how much he despised and loathed his kin by setting up a dogfight for the fortune and revealing that Annabelle West is to be the sole beneficiary. However, to claim the inheritance, Annabelle must spend the night in the house with the rest of the family and be deemed sane the next morning. Should she be unable to, West has a second reel of film in which he names the secondary heir. As the group prepare to turn in for the evening, they are startled by the arrival of Dr. Hendricks, a psychiatrist at a nearby sanitarium who warns them that a deranged, disfigured patient who believes himself to be a cat has escaped from the institution. Allison urges the others to keep the revelation from Annabelle for fear of frightening her, but Susan breaks the news to her. However, Annabelle seems unbothered. Allison requests Annabelle meet her in the library, but Annabelle is startled when, while turned away from her, Allison vanishes behind a
false bookcase. Annabelle alerts the others about the incident, but each have varying levels of suspicion regarding her claims, especially when taking into account the family's history of mental illness and delusions. Paul subsequently confides in Annabelle that he has discovered secret passageways beneath the mansion, and that he suspects Allison and Mrs. Pleasant may be hiding something. Later, Susan supplies Annabelle with a gun for self-protection. Paul and Annabelle subsequently locate a precious West family heirloom—an extravagant gold necklace—in the house, frozen in a block of ice. Later, when Annabelle falls asleep, the trenchcoat-clad killer enters her room through a trapdoor and steals the necklace from her throat, awakening and terrifying her in the process. Her screams again alert the others, who rush to her bedroom. The group is initially suspicious of Annabelle's claims, until Paul locates the trapdoor and, upon opening it, discover's Allison's corpse, her bloodied head wrapped in a pillowcase. When Paul goes to explore the basement tunnels, he is struck on the head by the killer, but survives. Shortly after, Susan is murdered by the killer in the dining hall while trying to locate West's second film reel. While Paul also proceeds to search for the reel to uncover the identity of the secondary heir, Annabelle arms herself with the gun given to her by Susan. On the staircase, she is confronted by the killer, who wrestles the weapon from her before dragging her into a room in the basement. A struggle ensues, during which Annabelle grabs at the killer's disfigured face and removes his rubber mask, showing the culprit to be Charlie. Charlie reveals that he is the secondary heir to West's fortune. Dr. Hendricks appears, and Annabelle runs to him for safety, but he reveals himself to be aiding Charlie. Charlie and Hendricks chain Annabelle to a medical chair in the basement. Charlie momentarily returns upstairs, where he is attacked by Paul and a fight ensues. Mrs. Pleasant hears the commotion, and enters the parlour, where she shoots Charlie to death. Paul and Mrs. Pleasant then rush downstairs to save Annabelle, who is about to be mutilated and killed by Hendricks; they enter moments before he is about to stab her, and Paul shoots him dead. At dawn, an ambulance arrives to remove the bodies, Harry and Cicily depart the mansion. Meanwhile, Mrs. Pleasant prepares tea for Annabelle and Paul as they proceed to view West's second reel, in which he names Charlie the second heir. ==Cast==