In a period where sea monsters abound, those who bravely hunt them are revered as heroes. Young Maisie Brumble looks up to her monster slayer hero, the beloved Jacob Holland. However, Jacob has been having moments of self-doubt lately. Thus, Maisie decides to sneak into his ship as he sails to fight off monsters anew. This time, Maisie becomes an unlikely teammate, cheerleader, and inspiration for him.