Late at night in early 1900s England, wealthy and elderly Ella Venable is murdered in her manor house by Andrew, the butler, in collusion with Walter Venable and Clara the maid. Tabitha, Ella's tabby cat, witnesses the crime. The murderers resolve to kill her. Walter forces Ella to sign a will that leaves everything to him. However, her original will — which left Walter nothing — remains hidden in the attic. Walter needs to find and destroy this original will to secure his inheritance.
Inspector Rowles and Michael Latimer are called to the house to investigate Ella's disappearance. Beth Venable, Ella's favorite niece, is invited to stay at the house. Beth and Michael are old friends from when she lived with Ella, and they become increasingly close.
While searching for the will, Walter has an accident in the structurally unsound attic, followed by an encounter with Tabitha which causes him to have a heart attack. Bedridden and unable to continue the search for the will, he invites his brother Edgar Venable; Edgar's criminal son, Jacob Venable; and Jacob's wife, Louise Venable, to the house. He promises them a share of Ella's money if they find her original will and kill Tabitha. They set a cage trap baited with mice, with which they capture Tabitha. They put Tabitha in a sack and send Andrew to drown her, but they forget to tie the sack shut, so Tabitha simply runs out when Andrew drops it. He pursues her into the swamp but falls in and drowns.
Beth is baffled that everyone is so afraid of a harmless domestic animal like Tabitha, while Michael says their fear may stem from guilty consciences, making him suspect they are culpable in Ella's disappearance.
Clara, frightened by the sight of Tabitha, falls down the stairs and dies. Jacob continues to search the attic for Ella's will but, fearing Walter may not keep his part of the bargain, turns on his uncle. He lets Tabitha into Walter's bedroom. When Walter sees the cat, he has a second, fatal heart attack. His will leaves everything to Edgar.
Beth, Michael and Inspector Rowles accuse the remaining villains of conspiracy. Edgar, now owner of the manor, orders them out. As they leave, Jacob sees Tabitha and pursues the cat onto the roof with everyone watching. He slips and falls to his death.
Edgar uses the distraction to go to the attic. He finds the original will hidden in the attic wall behind a painting of Tabitha. Tabitha herself appears and Edgar tries frantically to kill her, demolishing the attic in the process, and he is struck and killed by a falling beam.
Tabitha leads the police to Ella's body, which was buried in the swamp. The original will leaves everything to Beth. She tells Michael that she never wants to see the house again and asks him to take her away.
The house is sold and Tabitha watches from the courtyard as a new family — husband, wife, daughter and The Grandfather — move in. The grandfather complains that he will probably die of boredom living there, while the husband and wife talk of convincing the old man to change his will.
