Best friends Molly, a human, and Merrin, a mermaid, visit the Mermaid Moon festival on the night of Mermaid Moon, the one time each year mermaids can leave the sea.

"In a shabby house, on a shabby street, in the new capital of Madrid, Luzia Cotado uses scraps of magic to get through her days of endless toil as a scullion. But when her scheming mistress discovers the lump of a servant cowering in the kitchen is actually hiding a talent for little miracles, she demands Luzia use those gifts to better the family's social position. What begins as simple amusement for the bored nobility takes a perilous turn when...