As the story begins, our narrator is being sentenced to death by a panel of black-robed, pale-faced judges. Things aren't looking good: especially because he mentions that these guys are inquisitors. That means our story takes places during the Spanish Inquisition. And let's just say that during the Inquisition, sentenced to death means sentenced to horrible, painful, and super-torturous death.
(Quick side note: the story is narrated in the past tense, which means he's telling us all this after it's already happened.) So, he's fading in and out of consciousness and can vaguely tell that he's being carried down some stairs. When he wakes up, he's lying on the floor of a pitch-black cell. Slowly, he manages to get up and take a few steps forward. And once he finds the wall of his dungeon, instead of just freaking the heck out, he decides to measure its size. Ripping a piece of fabric from his gown, he sticks it into the wall to mark his starting place. Then he begins to walk, counting every step as he does.
Soon, though, he's overcome by tiredness again and falls to the ground. When he wakes up, after who knows how long, the narrator finds a loaf of bread and a pitcher of water next to him. Nice. He chows down, finishes examining the perimeter of his cell, and then decides to walk straight across it in order to measure its diameter. But shortly after he begins walking, he trips and falls at the edge of what he realizes is a deep pit. He's safe this time, but he knows now how the inquisitors are preparing to torture him. (Death by pit!)
Our narrator makes his way back to the cell wall and, soon enough, he falls asleep again. When he wakes up, he finds his cell has been illuminated: light! He can tell, then, that his original conception of the dungeon was totally wrong – it's not irregularly shaped, as he guessed, but perfectly square. Also, he's been tied down to a strange contraption, a sort of wooden rack that allows him use of only his left hand. Well, hey, that's something.
Next to him, there's a plate of heavily-spiced meat, which is being gnawed at by tons of rats that have come up from the pit. Gross. And…gross. Then our guy looks up: above, he notices that a picture of Father Time has been drawn on the ceiling, except that, in this case, his scythe has been replaced by a pendulum. Oh, and anti-bonus, the pendulum is slowly descending toward him.
As the pendulum comes closer, it somehow manages to get worse: he realizes that the bottom edge of the pendulum is razor sharp. This means it's meant to cut him in half. (Gulp.) After much worrying and more than one bout of fainting, he comes up with an escape plan. He rubs the spiced meat all over the bands that are tying him down and allows the rats to chew him out of captivity. He escapes just in the nick of time. Whew.
Actually, not whew. Though he may be freed from his bonds, he knows he is not free. The point is really driven home when the walls begin to glow and the shape of the room changes; he realizes, then, that he's slowly being pushed into the pit by walls that are caving in. Just when it seems like all is lost, when it appears that his only choice is to fall in the pit or throw himself against the burning hot walls, he gets rescued: he's pulled from the brink by the hand of General Lasalle, leader of the victorious French Army. The Inquisition and our narrator's torture are both finished. Okay, now whew.