Naïve 16-year-old Mamori Tokonome is accustomed to being teased at school for having an unfortunate surname that can also be read as "virgin." However, young Mamori will soon have to get used to being teased in other ways... Kidnapped during gym class, Mamori wakes up only to find herself stranded and under attack on the exotic island of Mermaid. Luckily, enigmatic fellow castaway Mirei Shikishima knows exactly how to take the lead—through a passionate kiss, Mirei unleashes Mamori's Exter transformation abilities, turning the innocent red-head into a battle-ready cutlass through the power of arousal.