The episode was based on the short story by Charles Beaumont published in the November 1959 issue of Rogue magazine.
A man who knocked on a door seeking sanctuary and found, instead, the outer edges of The Twilight Zone.While on a walking trip through post–World War I Central Europe circa 1919, David Ellington becomes lost in a storm.
The Devil had come to the village shortly after World War I to corrupt it, but Brother Jerome used the staff to imprison him.
Ellington waits until his guard falls asleep, then creeps down to the cell and sees that the door is held shut only by a staff that is within reach of the imprisoned man.
As he walks toward the exit, he begins to change, taking on the appearance of the Devil, before departing the castle in a plume of smoke.
Brother Jerome arrives, and sadly explains that the inability to recognize the Devil has always been Man's great weakness.
Charles Beaumont had originally envisioned that the monks would keep the Devil imprisoned by putting a cross in front of his cell door.