10-year-old Firmin Cailloux has been waiting 3 years for his explorer parents to return from at sea. He lives with his aunt in a small cockle fishing village but finds it hard adapting to this humble life. One foggy day, he spots a huge creature in the waves, which is drawn to the sound of the lighthouse's foghorn. Firmin begins to communicate with it, but he keeps this to himself to save the creature from human greed.