The family of Henrietta Winslow gathers at her stately home, in anticipation of her seemingly imminent death. Thinking that the elderly woman has already passed away, real estate agent A. Gilmore "Gil" Smith and antique dealer Mr. Penny arrive at the Winslow estate, only to be refused entrance by gardener Eduardo and housekeeper Abigail Doone. While Gil and Penny watch from outside, the invalid Henrietta appears before her family to read her will. She bequeaths her niece, Myrna Hartley, $100,000, and Myrna's husband Montague and stepson Richard $10,000 each. She then gives her granddaughter, Margaret Gordon, and grandson, Stanley Grayville, $100,000 each, while leaving her home and the remainder of the estate to her favorite granddaughter, Elaine Winslow. Outside, Gil begins to cough because the horde of cats Henrietta has living on her estate have triggered his allergies, and thus discovered, he and Penny are invited into the house. An upset Henrietta realizes that Montague has sent for Gil to sell the estate, and she tells the real estate man that she will never sell. After Gil saves Henrietta's life when he realizes that her milk has been poisoned, she reveals the last, unread portion of her will: No one will get any money until Abigail, the guardian of the estate, has died. Henrietta is then murdered while cremating a poisoned cat. When a black cat suddenly appears, an hysterical Abigail tells Gil and Elaine that it symbolizes death. Convinced that Henrietta's death was accidental, the family gathers together in hopes of settling the estate, while Gil tries to convince Abigail that her life is in danger. The housekeeper orders the family to leave the estate, but they refuse and send Montague into town to seek legal representation. Richard, however, discovers his father is having an affair with Margaret and threatens to go to his beloved stepmother unless he leaves his mistress. Penny, meanwhile, finds an unconscious Abigail while searching for antiques. Gil and Elaine question the injured housekeeper, who finally begins to realize the threat on her life. That night, Gil tells the family that there must be secret passageways in the house that his hypothetical murderer must have used. After numerous false alarms throughout the night, Abigail is discovered hanged in her room, and Gil finally manages to prove that the housekeeper was murdered. Myrna is seemingly attacked in the same manner, but is saved by Gil, Montague and Richard. Believing him to be the killer, the men chase after Eduardo, who is then killed by Myrna. Later, Elaine accuses Myrna of the killings and of staging her own attack in order to gain her inheritance and thus keep her husband. Myrna knocks Elaine out, then hides her body in the closet just as the men return. As Myrna prepares to burn Elaine to death in the crematorium, Gil bursts in and saves her. The black cat then knocks over a candle, setting Myrna afire. With the murderer uncovered, Gil and Elaine are finally united, and she agrees to sell the estate.