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 lives alone in a modest apartment, keeping to himself and managing his solitary routine with quiet efficiency. That changes the night he discovers a merman lounging in his bathtub. Wakasa is gorgeous, charming, and completely unbothered by the fact that he's now a permanent fixture in Tatsumi's bathroom. The merman settles in without asking permission, treating the tub like his personal sanctuary while Tatsumi reluctantly accepts his new reality. What follows is an odd-couple dynamic that spirals into increasingly ridiculous situations. Wakasa's casual arrogance clashes constantly with Tatsumi's practical nature and compulsive need to manage household affairs. The merman wants nothing more than to soak and look beautiful. Tatsumi wants his life back. Neither gets what they want, but somehow they keep circling each other, falling into an unexpected routine. It's comedy built on mundane domesticity mixed with the absurdity of sharing a bathroom with a mythical creature. The show thrives on the contrast between Wakasa's vanity and Tatsumi's exasperation, finding warmth in their accidental cohabitation.