When a sea-dwelling intruder takes over his bathtub, Tatsumi, a calm and collected teenage boy, must adapt to sharing his small Tokyo apartment with the lively and self-absorbed merman, Wakasa. As these two contrasting individuals navigate daily life together, unexpected appearances from Wakasa's oceanic pals—a humanized octopus, jellyfish, and hermit crab—add to the chaos. Get ready for a full house in Tatsumi's bathtub!