Tadashi Yamaguchi, a shy merman meets a cold and distant vampire Kei Tsukishima. Relationships between different species of monsters aren't allowed in their world. How will this affect their love? What strain will this put on their relationship? Will they give up on love and stay friends only?

On your 16th birthday, you get the name of your soulmate on your arm. Tadashi Yamaguchi often scribbles and doodles on his best friend's arm. Kei Tsukishima is that best friend and he wakes up with Yamaguchi's name on his arm.