Once, there were two best friends. One was from the sea, a mermaid named Merrin, and one was from the land, a human named Molly. Merrin has never been to Molly's house; how can she visit when she is a mermaid, and has to stay underwater? But then comes the night of the Mermaid Moon, when sea creatures can fly through the air! And this year, Merrin is finally old enough to join in. Together, she and Molly visit the Mermaid Moon festival, eat ice cream, and ride on a swing in Molly's garden. But trouble arises when it is time for Merrin to leave. If she doesn't get back home before the moon sets, the Mermaid Moon magic will disappear from the earth forever.