They are meeting at the carnival and Fortunato is drunk.
They are on there way to the catacombs, for more "wine". Montresor is getting prepared for his revenge.
They arrive to the catacombs and Montresor is looking for the perfect little spot for his revenge.
Montresor found the perfect spot he was looking for, a little cave area.
Montresor Got Fortunato in the cave area, trying to lock him in chains so he can start his revenge by stacking stones up so he becomes trapped in the little cave.
Now that Montresor has accomplished his revenge towards Fortunato he places the last stone in place and then processes what he had just done. Now Fortunato is stuck in the wall and will never get out. During the process of him locking Fortunato in the little cave, he thinks it is a joke and that they will laugh about this later on but what he didn't know was that there was no later on for him.