What happens when an intruder from the sea stakes claim to your bathtub? A cool yet demure teenage boy named Tatsumi must learn to live with the self-obsessed and playful merman, Wakasa, in a small Tokyo apartment. These two dissimilar young men will learn what it takes to deal with one another in their everyday lives. Of course, impromptu visits by Wakasa's ocean friends―an anthropomorphised octopus, jelly fish, and hermit crab―do not make things easier. Welcome to Tatsumi's bathtub. It's getting crowded.