Hiiragi Seichi is a fat, ugly high schooler who lives a boring life of getting bullied… However, one day all that changes. A mysterious voice suddenly rings out over the school’s PA system and sends them all to another world! Various challenges swoop down on him! When nearly dying of hunger, he finds the Fruit of Evolution! Upon eating it, a miracle occurs! Now Seiichi has it made in this new world with great looks, and amazing powers! ...Or so he thought when a gorilla appears!

After Seiichi defeats a gorilla-like monster, a Kaiser Kong, she proposes to him. The gorilla named Saria starts her new life together with Seiichi. While she is kind, innocent, and forthright, he still plans his escape from her since she’s a gorilla. After wandering into a cavern he finds a skull putting off a strange aura - the dark lord Xeanos. Seiichi begins to battle Xeanos, a creature as strong as most final bosses! However, disaster befalls Seiichi and Saria leaps in before him!

It looked like Saria had lost her life in battle, but after eating the Fruit of Evolution, she revives as a beautiful woman! Seiichi leaves the forest with Saria and heads to the royal capital so he can find out where his friend from school, Kannazuki-sempai, went. At the royal capital Seiichi tries to join the adventurer’s guild, but he’s greeted by abnormal people like a dark-skinned musclehead and sadistic noble lady! There Seiichi also meets the female adventurer, Artoria “The Calamity”, who has issues of her own.

Seiichi and Saria train under Artoria to become adventurers. During their training the three of them are teleported into a dungeon by a demon’s trap. The dungeon they find themselves in the stronghold of the Black Dragon God, the strongest member of the Demon Lord’s Army. Artoria gets separated from Seiichi and Saria in the dungeon and the giant Black Dragon God appears before her. Then that overwhelming threat attacks! Can Seiichi and Saria save her in time?!

When Seiichi rescues Artoria, she’s on the verge of death, so he faces the Black Dragon God himself! Though the frightening strength of the Black Dragon God poses a challenge, he manages to defeat it. Artoria feels depressed, blaming herself for dragging them into her calamity. She reveals she suffers from a curse that brings misfortune to those around her. After lamenting her fate, she leaves the others. Seiichi follows after her to try and cheer her up, but…

Seiichi’s attempts to cheer up Artoria somehow come out as a proposal. While their relationship grows awkward, Seiichi and Saria pass their tests and successfully join the adventurer’s guild. Meanwhile, a plot advances in the Kaiser Empire to enslave Kannazuki and the other students from another world and send them to Barbador Magic Academy. After heading out into the royal capital Seiichi runs into all kinds of people, and taking their words to heart he finally makes up his mind to face Artoria.