Something is calling from the sea caves near the Ovens—a low, haunting note that wasn't there last spring. When barge operator Barry reports hearing it with his engine off, Millie Mollusk and her crew of animal detectives set out to solve the mystery: Millie the careful note-taker, twins Phillip and Suzie who split fact-finding and timing duties, steady Goldie and Fred, and Bulu—a seagull with unstoppable confidence, a satchel full of "evidence," and a serious weakness for snacks and bread.

Following clues along real clifftop trails, the team tests theories, listens from three different lookouts, and even builds a working model to catch the sound in the act. Just when the case seems closed, a strange carved mark appears beneath the spray—one that matches a symbol from an old mystery. Is it connected? The team isn't sure yet... but their next adventure is already calling.