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.