The Latin epigraph essentially says that "here" was a place where torturers fed their thirst for innocent blood, but now that the place of torture has been destroyed, life and health will prevail.
The narrator recalls his situation: he finds himself bound and then unbound, then sentenced to death by the Spanish Inquisition (in the story, it is referred to simply as the "inquisition"). The narrator goes on to recall the trial, his judges, and all that happened to him, with figures carrying him down into some cavern:
These shadows of memory tell, indistinctly, of tall figures that lifted and bore me in silence down--down--still down--till a hideous dizziness oppressed me at the mere idea of the interminableness of the descent.
The narrator is to be tortured, mentally and physically. The narrator finds himself on the damp ground. He refuses to open his eyes for fear that he will open them only to find darkness which turned out to be the case: he is in total darkness. He then begins to feel his way around in the darkness to understand the dimensions of his dungeon, first by sticking close to the wall. After traveling the circumference of the dungeon, he decided to cross the middle. He falls and discovers a pit in the center, but avoids falling into it.
His plight continues as he faces another method of torture: the pendulum. He wakes up to find himself bound on his back as a razor sharp pendulum is slowly lowered down to him. The remainder of the story describes his slow, agonizing wait as he tries to free himself while the pendulum descends upon him.