The narrator is sentenced by the Inquisition and brought down to a mysterious dungeon. After attempting to figure out the nature of his prison, he falls down at the edge of a huge pit and faints. Not a great first act for our protagonist.
The narrator finds himself tied down to a peculiar contraption. He realizes that he's going to be cut in half by a huge pendulum descending slowly from the ceiling. This still sounds pretty terrible. But, wait! Thinking quickly, he gets a bunch of ravenous rats to eat through his bonds. He escapes the pendulum.
After the pendulum is pulled back up into the ceiling, the walls begin to glow hot and move in on him. The narrator is slowly pushed toward the edge of the pit. Just when things seem totally hopeless, he's saved (at the very last second – seriously) by General Lasalle and the victorious French forces. Ta-da!