On a stormy night, an elderly David Ellington warns that people may not believe him but he must explain what happened.
Ellington is on a walking trip in Europe after World War I and gets lost. He finds his way to a castle and bangs on the door. Brother Christophorus of the Brothers of Truth initially refuses to allow visitors but Ellington explains that he got lost and he needs help. Christophorus takes pity on him and lets the ailing traveler in, and goes to speak to his brothers. Ellington hears an unearthly howling noise and Christophorus returns to say that Brother Jerome, the head monk, will see him. Jerome tells Ellington that he must leave but as he goes, Ellington collapses, tired and exhausted.
Later, Ellington wakes up and hears the howling noise again. Wandering around the monastery, he finds a door bolted with a monk's crook. He goes to the barred window and a desperate prisoner begs him for help. The prisoner claims that the brothers are mad, and took him prisoner as he was kissing a girl. The girl had rejected Jerome, who took out his anger on the prisoner. Before the prisoner can explain further, Jerome arrives and escorts Ellington away, saying he must leave immediately. When Ellington protests that he was just talking to the man, Jerome says he talked to no man and is delirious. They hear the howling noise again and Ellington threatens to go the police. Left with no choice, Jerome decides to explain and says he's only told the partial truth. He says he has heard the howling for five long years but there is no man in the cell. The prisoner isn't a man, but the Devil himself.
It's clear that Ellington doesn't believe him and Jerome insists he isn't an ignorant fanatic. He had a degree in philosophy and traveled the world for forty years, but embraced a different point of view. Five years ago the family that owned the castle donated it to the Brothers, who are devoted to truth as a weapon against the Devil. Just after the Great War, the village below was peaceful so the Devil came with a program of corruption. He underestimated Jerome, who lured him into a cell. They use the Staff of Truth, the one barrier the Devil cannot pass, to keep him in the cell. Jerome explains that he saw the Devil always at places of chaos and destruction, and learned to recognize him no matter what form he took on. Ellington suggests that Jerome may have made a mistake but the brother notes that there have been no great catastrophes, war, or disease in the last five years since they imprisoned the Devil. Ellington says he believes Jerome and promises to keep his secret. Jerome believes him and says he can leave the next day.
Ellington returns to the cell and tells the prisoner what Jerome said. The prisoner scoffs at the claim, but Brother Christophorus arrives to take Ellington to his room. The brother locks him in but dozes off and Ellington takes the key and sneaks off. He goes back to the cell and the prisoner begs him to remove the staff. Ellington doesn't understand why the prisoner can't remove the staff himself, while Christophorus wakes up and finds the key missing. Ellington removes the staff and the prisoner steps out. Ellington gives him a cape against the weather but the prisoner points at him and causes him to collapse, paralyzed. Ellington looks on in horror as the prisoner strides away, his face transforming and horns appearing on his forehead. The prisoner steps to the window and then disappears in a flash of brimstone. Jerome and Christophorus arrive and realize what's happened.
Ellington explains to his housekeeper that he hunted down the Devil for decades, through many wars, and finally tracked him down. Now the Devil is locked in a room in his house, held by a miniature version of the Staff of Truth. Ellington leaves to make preparations to ship the Devil back to Brother Jerome, and warns the housekeeper to keep the door locked. After he leaves, the housekeeper hears a howling noise and reaches out for the Staff to see what it contains...