The unnamed narrator is brought to trial before sinister judges of the Spanish Inquisition. Poe provides no explanation of why he is there or of the charges on which he is being tried. Before him are seven tall white candles on a table, and, as they burn down, his hopes of survival also diminish. He is condemned to death, whereupon he faints and later awakens to find himself in a totally dark room. At first the prisoner thinks that he is locked in a tomb, but then he discovers that he is in a cell. He decides to explore the cell by placing a scrap of his robe against the wall so that he can count the paces around the room, but he faints before he can measure the whole perimeter.
When he reawakens, he discovers food and water nearby. He tries to measure the cell again, and finds that the perimeter measures one hundred steps. While crossing the room, he trips on the hem of his robe and falls, his chin landing at the edge of a deep pit. He realizes that had he not tripped, he would have fallen into this pit.
After losing consciousness again, the narrator discovers that the prison is slightly illuminated and that he is strapped to a wooden frame on his back, facing the ceiling. Above him is a picture of Father Time, with a razor-sharp pendulum measuring "one foot from horn to horn" suspended from it. The pendulum is swinging back and forth and slowly descending, designed to kill the narrator eventually. However, he is able to attract rats to him by smearing his bonds with the meat left for him to eat. The rats chew through the straps, and he slips free just before the pendulum can begin to slice into his chest.
The pendulum is withdrawn into the ceiling, and the walls become red-hot and start to move inwards, forcing him slowly toward the center of the room and the pit. As he loses his last foothold and begins to topple in, he hears a roar of voices and trumpets, the walls retract, and an arm pulls him to safety. The French Army has captured the city of Toledo and the Inquisition has fallen into its enemies' hands.