Located in the city center, the newly built Goryoukan Girls' Academy
with integrated middle and high school boasts its comprehensive facilites and
vast school grounds.

Despite its popularity, there is another side of this academy that's being
kept secret.

That secret is...

The training and formation of special forces "Fifth Force" to protect
the world from an invisible monster called "Obli" that tries to destroy the world.

The academy picked students with ability to fight those monsters and formed
five-people teams to carry out the mission.

And then, a newly appointed captain and 5 students have gathered here, in this academy.
(Roughly translated from game's description)