Science and magic exist side by side in this world. This is the peak of the science side with a curriculum for espers, Academy City. In this city where 80% of the population are students, the season has finally come when all residents are excited and fidgety! It's Christmas Eve. Amidst the city showing a cheerful hustle and bustle, ordinary boy who can't keep up with school, Kamijou Touma, also doesn't intend to miss this event! He was hard at work in his supplementary lessons A backward glance from the default misfortune-prone bastard, the silver-haired sister freeloading in his dormitory, Index, in the heart pounding scene different from the usual, activates a new gluton skill!? While Kamijou tries to desperately stop that, Misaka Mikoto taken in by the Christmas Eve atmosphere appears before him.
A massive underground labyrinth awaits in an alternate world of swords and magic. With a game-like system much more familiar than spells and magic circles, Beatrice has managed her magic and risen to the position of "strongest", but she still has her worries. "Bghohhh!! Bshrrrrmumble mumble" "B-Boo Boo! Stop! Stooooooop!! I'llI'll be crushed!? I-is my, ah, love being tested!?" Who is the legendary man named Boo Boo who, just by taking a nap, can overpower the Level Cap Holy Swordswoman? A thousand meter dragons and a proud elf girl will also show up, so what exactly is going to happen with these residents of another world!?