The most frightening moment in the story occurs at the very end when Mr. and Mrs. Smith hear the knocking at their downstairs door, late on a very dark night. From the details developed throughout the story, the reader infers it is their dead son Herbert knocking to come in. Herbert is not perceived here to be a ghost, but a living corpse who has come out of his grave.
The mood of this scene adds to the suspense. The darkness of the night is “oppressive.” A stair creaks. A clock ticks in the silence. The knocking at the door is “quiet and stealthy.” Mr. Smith goes down the stairs with the light of a burning match that soon goes out. When the knock is repeated, Mr. Smith flees back to his room, terrified.
After the knocking has continued and grown louder and more insistent—even angry-sounding—Mrs. Smith rushes downstairs to the door and struggles with the bolt. She tries desperately to let her son in. Upstairs, Mr. Smith struggles to find the monkey’s paw to make his final wish, that Herbert go back to the peace of the grave.
The moment of greatest fear is then achieved when Mrs. Smith finally manages to get the door open: No one is there.
The most frightening single moment in "The Monkey's Paw" occurs in Part III when Mr. White goes downstairs to get another candle. At his wife's insistence, he has wished for his son Herbert to return. But nothing has happened. He feels a sense of "unspeakable sense of relief at the failure of the talisman." His wife apparently gives up hope and has come back to bed. Since Mr. White does not have a candle to see his way down the stairs, he is holding a lighted match. Then:
At the foot of the stairs the match went out, and he paused to strike another, and at the same moment a knock, so quiet and stealthy as to be scarcely audible, sounded on the front door.
Why should the knock be "so quiet and stealthy as to be scarcely audible"? It suggests that the person outside must have some guilty secret. If the knocker is Herbert returned from the grave, then he may have been trying to remain as inconspicuous as possible all the way from the cemetery to his parents' home. Why? Because he was well aware how horrible he looked after being mangled by machinery and decaying in his grave. The knock seems to contain a confidential message that Herbert wants to come home and live with his parents but does not want anybody else in the world to see him or know he is there.
Mr. White's reaction to that quiet and stealthy knock suggests how frightening that moment was.
The matches fell from his hand and spilled in the passage. He stood motionless, his breath suspended until the knock was repeated. Then he turned and fled swiftly back to his room, and closed the door behind him.