Meri Murphy is not a mermaid's daughter. At least, she doesn't think she is.

What she does know is that her father is the town drunk who tells anyone who will listen that his wife hadn't left him fourteen years ago. No, she was a mermaid, and returned to the sea to save her people and all of Ireland from an evil tyrant.

Meri wishes he would stop telling that story.

Everything changes during a school trip to the Cliffs of Moher. First, Meri learns that mermaids were once called merrows, and their children are the merrowkin. While at the Cliffs she glimpses a golden tower on the waves, and learns about a city that lies at the bottom of the bay between Ireland above and the land of gods below, a barrier between the two worlds that rises once every seven years. Suddenly, her father's stories aren't the strangest things she's heard.

Accompanied by her friend, Aodhan, Meri embarks on a quest to discover what really happened to her mother, and hopefully bring her home.