Hyoudo Riki is just an ordinary guy who likes to wear a tiger mask while Kung-Fu fighting, which means he fits in real well at BATTLE ROYAL HIGH SCHOOL!
Unfortunately for Riki, he's the doppleganger in our world of Byoudo, Master of the Dark Realm, and when a dimensional gate opens between the two worlds, the evil Fairy Master tricks Byoudo into making an attempt to conquer our beautiful, and better lit, world.
Now poor Riki is semi-possessed by his opposite number, and the minions of the Fairy Master are mutating his classmates into hideous lustful monsters. To make matters worse, all the inter-dimensional commotion has attracted the attention of a sword-happy exorcist and the heroic Zankan of the Space-Time Police!
Which all means that Riki is about to take one bitch of a mid-term. The course is survival, and forget "Pass/Fail". The test is graded "Live/Die".