Tani dreams of fulfilling her destiny as a Serpent Rider, fighting alongside an elemental serpent to defend her village against monsters. More than anything, she wants to protect her little sister Ligaya, the village princess who's tasked with memorizing their community's history, including the tale of Great Bakunawa, the most powerful and feared sea serpent that swallowed six of the world's seven moons. After a devastating monster attack, Tani can no longer wait patiently for her bakunawa egg to hatch. She makes a desperate deal with a stranger who promises her a serpent of her own. But when her gamble puts her sister in major danger, Tani must find a way to save Ligaya before she's lost forever. All the while, in the depths of the sea, Great Bakunawa is stirring, threatening to devour the last moon and plunge the night sky into eternal darkness . . .