The narrator, Zaleukos, a native of Constantinople, studied medicine in Paris and following his father's death, develops a successful business as a travelling merchant and healer.

Arriving in Florence, he sets up shop as usual. But after he has been in town a few days, he finds a small handwritten note from an unknown stranger, asking to meet him at midnight on the Ponte Vecchio. Curious, he goes to the bridge in question, but ends up tussling with the stranger and comes away with the man's red cloak.

Attempting to lure the cloak's owner, by putting it on sale for an extortionate price in his shop a new incident soon occurs which results in him being passed another mysterious note, again asking for a meeting on the same bridge as before. This time, the purpose of the meeting becomes clear... but it is a very odd request indeed, although financially well rewarded. But as Zaleukos is drawn into the intrigue, his mission takes on a far more horrible character, and before long he has committed an awful crime and is in imminent peril of his life.