Renaissance artist, Roberto Robertini, and his young apprentice Felicia, are forced to leave Florence in a hurry. They think themselves lucky to find passage on a ship bound for England, unintentionally becoming passengers number 12 - and unlucky 13. The passage is made unpleasant by one of their fellow passengers, who poisons the trip with sneers, slights, innuendo and insult to all the other passengers. What with this unpleasant fellow traveller, and pirates too, the trip could not get much worse. Then more trouble falls: murder by rat poison of the poisonous Edward Marsh. The two artists must search out the killer to free themselves and the other passengers from suspicion.