The "beautiful girl sexy battle action" anime is set on five artificial islands.
Mamori is a "mermaid" transferred to one of those islands. When Mamori is
attacked, a newly transferred girl named Mirei saves her. The enemy doesn't
stop, however, and the two are soon cornered. Just when they think all hope
is lost, Mirei kisses Mamori, and Mamori turns into a sword. Mirei then wields
the sword and launches a counterattack against their enemies.