At the edge of the flat Earth, a strange fire in the sky burns, altering the balance of the elements and threatening all life on Horizon Island. Matthew Marcheford experiments to determine the cause, while the growing animosity of the manticores and the arrival of more ships from Europe jeopardize the colony's fragile peace. When Matthew's fiancee, Catherine Parris, is betrayed and abandoned deep underground, the secrets she uncovers there may be Horizon's last hope.

King Philip of Spain, determined to control quintessence, hires a Jesuit priest named Ramos de Tavera. Ramos unlocks secrets that would give Philip the power to conquer the world, but he hesitates, knowing the king's ambition and brutality. Instead, he seeks the advice of Princess Elizabeth, imprisoned in the Tower, and his faith in church and country begins to falter. To protect his family from the king's wrath, Ramos must work together with Matthew and Catherine across thousands of miles to rescue Elizabeth and save the Horizon colony from destruction.