The protagonist is a psychotic chuunibyou version of Haruhi in a fantasy world. Most of his delusions turn out to be true, but he isn't self-aware enough to realize it. If that sounds absurd, that's because it is - but man, what a ride!
Although the story essentially relies on one joke, it remains consistently hilarious, and later on, in a strategy similar to One Punch Man's, we see a bigger focus on the side characters, so whenever the protagonist makes an appearance with his chuuni antics, it's always a treat.
The manga also pokes fun at isekai tropes which should get a laugh out of any seasoned reader of the genre.