The narrator traces out a long, long journey moving all the way from China to the tropical Caribbean island of St. Hubert (not a real island, btw).
But who or what is taking this journey? The Black Death! Or, to put it in more scientific terms: the bubonic plague.
Unfortunately, the plague reaches the island at the exact same time that the government decides to fire the local rat catcher. The plague is spread by the fleas on rats, so getting rid of the rat catcher helps the disease spread like wildfire.
Scientific knowledge is limited on St. Hubert. Everyone there assumes that the plague can never come to St. Hubert because it doesn't exist in tropical climates. This is very much not true.
When people start dying, the local officials deny that their St. Hubert is infected with plague. They say it'll all just blow over.