In the opening scene, set on a 1880's Victorian-era private island somewhere on the Hudson River, a young couple, Robert and Ada, are having a picnic near an old house when Robert wonders off and gets attacked and hacked apart by a wild-eyed hunchback man. Ada goes off to look for him when she too is attacked and killed.Some time later in Philadelphia, Vicky receives a letter from a lawyer explaining about her father's recent passing and she wants her husband, Richard, to travel with her to New York City to meet with Dobbs, her father's estate lawyer. Before leaving, Vicky and Richard meet with Father Walter, an old friend to ask for money since Richard is broke. It is implied that Father Walter and Richard had a secret sexual tryst sometime back.In New England, Vicky's younger sister, Elizabeth, received a similar letter and tries to get her lazy husband, Donald, to listen about this turn of events.In New York, Veronica, the youngest sister of Vicky and Elizabeth, receives a letter from Dobbs and tells her emotionally distant and barren husband, William, about her sisters arriving for a visit. In New York, while the men go off to a men's club, the women have dinner at a local restaurant where they talk about what may lay in store for them since they have not seen each other for several years.The next day, the three couples meet in the office of Lawyer Dobbs, a decrepit lawyer of their late father who has willed that upon his death, his three daughters, having married, must spend three days and two nights at his old house on Crenshaw Island on the Hudson where the money and possessions will get divided up among everyone.Upon arriving on the snowy and desolate island, Vicky, Rich, Liz, Donald, Veronica, and William are met by Hattie and Martha, the two housekeepers, whom lead them on a snow-covered path to the house. Accompanying them is Colin Trask, Hattie's mentally retarded hunchback brother (and the same killer from the opening scene), who works as a handyman on the property. Colin sees a white rabbit in the snow when he attacks it and eats it alive, forcing Martha to pull him away from his gruesome doings.During an uneventful evening, the two housekeepers tell the three couples about their dinner plans and about their employer, the girls father Mr. Crenshaw, whom was always out of the country on business and only lived inside the house five times in the past 30 years. Liz becomes worried that something is not right about Hattie or Martha, but Donald tells her that she's just imagining things.Later that night, when Veronica refuses William's desires for sex, he forces himself upon her and slaps her when she protests. Meanwhile, Vicky begins to confide in Richard about problems with this whole situation, but still wants to spend the time with him the best she can. When they hear a noise, Richard goes out of the bedroom to investigate and gets attacked by a shadowy, robed figure who brutally stabs him to death and throws him off the second floor balcony.In the morning, everyone is disturbed by Richard's mysterious death, but with no way of contacting anyone, they will have to wait until Lawyer Dobbs will arrive the following day with the final paperwork for the Crenshaw estate. Colin begins to try to comfort Vicky about her husband's murder, but she is put off by his ugly appearance despite his friendly attitude. When Donald ventures down into the cellar to get some more firewood for the fireplaces in the house, he gets attacked by the same robed figure who ties him to a table and disembowels him with a saw. Liz soon goes missing too, and when dinner is served that night, her severed head is discovered on a serving tray that Hattie and Martha serve for them.Now in fear that a killer is among them, Vicky, Veronica and William hold themselves up in a room to wait until morning. When William decides to venture out to have a talk with Hattie and Martha about what is going on, he is lured to the cellar where the robed figure attacks him by pinning him to a wall with a pitchfork and then disembowels him too. In the morning, Colin discovers William's dead body in the cellar as well as all the other murder victims and flees outside where he and Martha get attacked by the robed figure who stabs Martha to death and chases Colin back inside the house where the killer throws lamp oil onto him and sets him afire.Vicky and Veronica hear Colin's screams and venture outside their room when they come face to face with Hattie, whom is the killer all along. Hattie explains to the two surviving sisters that she is their long-lost half-sister whom was the product of their father's one-time infidelity and has been killing everyone so she can lay claim on the estate for herself. Just as the murderous Hattie is about to stab the ladies with a meat cleaver, the horribly burned Colin revives and lunges at Hattie in which she gets stabbed in the head with her own cleaver and they both tumble down the stairs to the ground floor, both dead... leaving Vicky and Veronica as the only survivors of the massacre.... and having to face Lawyer Dobbs when he shows up minutes later.
Output: Question: Why did Hattie plan to kill the others and blame Colin?, Answer: She wants to lay claim to their father's estate.