A merman in your bathtub sounds ridiculous until you realize the show is not interested in explaining anything, just in letting two people who have nothing in common slowly become each other's routine. Wakasa is vain and lazy, Tatsumi cannot stop fussing over him like he's been waiting for a reason to care about something. The comedy lives in that friction, in the domestic mundanity of feeding a mythical creature leftovers and arguing about shower schedules.

Tatsumi is a high school student who lives by himself. Walking home from school one day, he comes across a man collapsed near a lake and discovers that the stranger is actually a beautiful merman named Wakasa. Wakasa's home has become too polluted to live in, so Tatsumi offers his own bathtub as a place to stay. Wakasa is somewhat self-centered but cute, while Tatsumi is cool-headed with a meddling streak, and their mismatched personalities set up a light comedy of shared living. Wakasa's aquatic friends Takasu, Mikuni and Maki also drop by without invitation, curious about human life, and Tatsumi has to keep up with his everyday routine while indulging their playful behavior. Now and then he finds himself enjoying their company as an unusual friendship takes shape in these short fantasy slice-of-life episodes.