Monster Portals appear alongside spatial distortions.

Unlike dungeons, these portals continuously spew out monsters for a fixed duration.

Just when you think you've finally defeated all the monsters, the master of the portal—the Boss—appears.

Unlike other monsters, this boss is lethal in every way, causing significant casualties.

There are two main reasons for such high casualties. First, the portals operate on a time limit concept.

A monster portal will never disappear until the time limit expires; you must endure until the end to win.

Second, even if you try to stall for time so the portal closes automatically, once the master known as the Boss appears, the portal will not close unless that master is defeated.

Additionally, higher-ranked portals take more time and require stronger manpower to clear.

Academy

Marc Academy trains students to become heroes or guardians to block monster portals.

At the academy, regular professors send students to the battlefield under the pretext of field experience.

Marc Academy is not the only academy; others exist as well.

This story covers what happens after you arrive at this academy.

The academy ranks are as follows:
F E D C B A S SS SSS X

Now... your story begins. Have a good day. Well then... farewell.