When housekeeper Ellen Creed receives a letter demanding the immediate removal of her two mentally deranged sisters from the house where they have been living, she asks her employer, retired actress Leonora Fiske, to allow the two to visit her at Miss Fiske's house, which is located in the marshes of an estuary of the Thames river. After Miss Fiske consents, Ellen travels to London to fetch her sisters. In her absence, a rogue named Albert Feather comes to the house, looking for his aunt Ellen. Informed by Miss Fiske that Ellen is in London, Albert divulges that he desperately needs twelve pounds to replace funds he embezzled from the bank where he is employed. Albert's tale strikes a chord of nostaglia in Miss Fiske, and she unlocks her oven, which now serves as her safe, and hands the money to him. Albert then departs, and several days later, Ellen arrives with her sisters: the fractious and defiant Emily and the simpleton Louisa. Six weeks later, Miss Fiske, exasperated by the chaos and disorder that Emily and Louisa have wrought upon her home, asks Ellen to send them away. When Ellen begins to argue with her employer, Miss Fiske fires her and orders her to leave immediately with her sisters. Later, Ellen apologizes to Miss Fiske and begs her to relent, claiming that Emily and Louisa will be institutionalized unless she cares for them. When Miss Fiske turns a deaf ear to her pleas, Ellen tells her sisters that Miss Fiske has agreed to sell her the house and is going away and then sends them out for a picnic. As Miss Fiske sits alone at her piano playing her favorite tune, "Tit Willow," Ellen sneaks up from behind and strangles her. Some days later, two nuns from the local priory visit the house to borrow some oil, and Ellen tells them that Miss Fiske is away on an extended voyage. Ellen then sends Lucy, the maid, to the shed to retrieve some oil, and there, Lucy finds Albert hiding. After ascertaining from Lucy that Ellen, who has never met him, is unaware of his previous visit, Albert knocks on the front door and introduces himself. When Ellen tells him that Miss Fiske is traveling, he confides that he is wanted by the police for embezzlement. Ellen agrees to let him stay at the house until she can arrange for his passage out of the country. After everyone has gone to bed that night, Albert picks the lock of the oven-safe and discovers that it has been bricked up. The next day, Albert tries to seduce Lucy as she cleans Miss Fiske's room, and when Lucy resists him, she knocks over a case containing Miss Fiske's best wig, causing them to wonder why she would leave it behind. Albert's puzzlement deepens the next day when Emily blurts out that Miss Fiske sold the house to Ellen, and later, the nuns visit and comment that Miss Fiske never mentioned that she was leaving. His curiosity piqued, Albert opens a letter addressed to Miss Fiske and learns that the bank is questioning the signature on her recently cashed checks. Becoming suspicious, Albert studies Ellen's expression as she opens the letter from the bank. After urgently penning a response, Ellen hurries off to post her letter and Albert uses a mirror to read her reply on the ink blotter. Upon discovering that Ellen has signed her reply "Leonora Fiske," Albert realizes that Ellen has been impersonating Miss Fiske and deduces that she has killed her. Enlisting Lucy in a scheme to blackmail Ellen, Albert begins to dismantle the bricks in the oven. That night, Ellen demands that Albert leave the house by morning because Miss Fiske is coming home. In response, Albert taps his pipe on the oven and says that he dreamt that Miss Fiske was dead. As Ellen tosses in her bed later that night, she hears "Tit Willow" being played and slinks down the stairs to see a strange woman seated at the piano. Although it is only Lucy wearing a wig, Ellen screams and faints. The next day, Albert forces the tormented Ellen to confess her crime and then demands money for his silence. At that moment, the nuns knock at the door to warn Ellen that the police are in the vicinity searching for her nephew. Overhearing their conversation, Albert flees, and Lucy, finally realizing the depravity surrounding her, screams and runs away. Soon after, Emily and Louisa report seeing Albert being led away by some men. Deciding to turn herself in to the police, Ellen kisses her sisters goodbye and disappears into the marsh.