While on a seemingly peaceful summer vacation trip to her grandparents' home on a small island, energetic preteen Shiina Tamai nearly drowns in an ocean swimming race. Before blacking out, she discovers a strange alien starfish that brings her back to shore. She befriends the creature, naming it Hoshimaru and taking it back to the city with her. What Shiina doesn't know is that her cute pet may end up doing her more harm than help.