Mellow and responsible Tatsumi has gotten pretty uesd to his new roommate—the energetic and needy merman, Wakasa—who has taken what seems like permanent refuge in his bathtub. While regular visits from Wakasa's undersea friends and Tatsumi's affectionate little sister have become commonplace, the bathrub suddenly seems a little less safe when a shark appears. Is their hungry new acquaintance friend or foe? Whether it's testing new bubble bath, hosting a butler cafe, or training for the Olympics, it's never a dull day in Tatsumi's bathroom.