The story begins with the narrator receiving a death sentence from the court of the Inquisition for an unknown crime.
He awakens in darkness, wondering how much of what he remembers was a dream and how much was reality.
Fearful, the narrator again faints, and after he awakes for the second time, he begins to explore the dungeon while wondering what his fate will be.
The narrator then decides to cross the center of the room, moving with increasing confidence until he fortuitously trips and lands prone at the edge of a circular pit. By dropping a stone from the masonry at the edge of the pit, he discovers that the pit is very deep and filled with water at the bottom. He hears a door closing and realizes that he has narrowly escaped his death.
However, the water is drugged, so he again falls asleep and wakes up to find himself in a slightly different situation. He is securely bound on his back by a long strap that has been wound around his body and attaches him to a wooden framework so that he can only move his head and, to a lesser extent, his left arm.
The ceiling, meanwhile, is thirty to forty feet above his head and plated with metal. One of the plates features a typical painting of the figure of Time, although Time appears to be holding an image of a pendulum rather than the more commonly associated scythe.
For some thirty or sixty minutes, he concentrates on scaring the rats away from his food, but when he again looks at the ceiling, he sees that the arc of the pendulum's swing is about a yard larger, that the pendulum is swinging faster, and that, most importantly, the pendulum has visually descended. The pendulum, he now sees, has a razor-like edge of steel and is attached to the ceiling by a brass rod.
As the blade swings closer, he waits in a frenzied anguish for the blade to begin fraying his robes and vainly struggles to free his arm.
Nevertheless, he manages to pull his thoughts together for long enough to find a potential solution. He spreads the remains of the oil and spice from his food onto the strap and lies still so that the hungry rats swarm his body in order to eat away at the strap.
The cell heats up further and begins to flatten into a narrowing diamond so that the narrator will eventually be forced into the pit. The narrator clings to the heated walls but is ultimately forced to the brink of the pit and screams in despair. As he is about to fall in, however, he hears voices and trumpets as the walls return suddenly to their normal shape. Having just led the French army into Toledo and beaten back the forces of the Inquisition, General Lasalle rushes in and catches the fainting narrator by the arm before he falls into the pit. The ordeal is over.