A handsome merman named Wakasa has made his home in the bathtub of Tatsumi, a high school student who otherwise lives alone. Their friends gather around the tub to sing karaoke, dress up, and play all kinds of inappropriate games...love is in the air, until suddenly their precious tub gets caught in an explosion! Tatsumi's sketchy uncle, Hisatora, appears in this volume, along with his collection of self-formulated bath salts! The fourth volume of this popular bath comedy brings the story to the boiling point!