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.