In Venice, magic is not unusual. Merle is apprenticed to a magic mirror maker, and Serafin—a boy who was once a master thief—works for a weaver of magic cloth. Merle and Serafin are used to the mermaids who live in the canals of the city and to the guards who patrol the streets on living stone lions. Merle herself possesses something magical: a mirror whose surface is water. She can reach her whole arm into it and never get wet.

But Venice is under siege by the Egyptian Empire; its terrifying mummy warriors are waiting to strike. All that protects the Venetians is the Flowing Queen. Nobody knows who or what she is—only that her power flows through the canals and keeps the Egyptians at bay.

When Merle and Serafin overhear a plot to capture the Flowing Queen, they are catapulted into desperate danger. They must do everything they can to rescue the Queen and save the city—even if it means getting help from the Ancient Traitor himself.