While walking home from school, Tatsumi stumbles upon a man collapsed near a lake, only to discover that the stranger is a charming merman named Wakasa. Taking pity on Wakasa due to his polluted underwater habitat, Tatsumi generously offers his bathtub as a temporary home. This act spirals into a whirlwind as Wakasa becomes Tatsumi's lively housemate, joined by his aquatic pals—Takasu, Mikuni, and Maki—who frequently stir up mischief in Tatsumi's life. Despite Tatsumi's amusement at their inquisitive nature and whimsical behavior, he must navigate the chaos to balance his ordinary routines and newfound friendships.