High school student Yugo is helping his childhood friend Miu move into his house when he stumbles upon a box he had long forgotten. Marked with dire warnings like do not open and sealed, the box contains the remnants of his middle school obsession: stacks of self-written fantasy novels and elaborate setting notes, all penned under the chuunibyou pseudonym Tenka Issetsuna. His secret trove of embarrassing black history, filled with original magic systems and overpowered characters, is immediately discovered by a laughing Miu. Before Yugo can process the humiliation, a sudden storm strikes, and a monstrous creature appears before them.

A dark-skinned, scantily clad beauty emerges and destroys the monster with a blazing firearm. She is Fei Iburedo, the heroine from Yugo's abandoned stories, and she calls him Master and her creator. However, Fei is not alone. Yugo's unfinished fantasy world, the Black Chronicle, is beginning to leak into reality, and not all of his creations are friendly. He soon finds himself caught between the demands of his own characters, who seek an ending to their story, and the chaos of living with his childhood friend and a fictional battle-hungry heroine who wants his affection.

The setting is modern day Japan, disrupted by the physical manifestation of Yugo's fantasy lore, including magical artifacts and creatures from his manuscripts. Yugo is a relatable protagonist forced to confront his cringeworthy past. Fei is the loyal and affectionate battle gunner who wields the Crimson Blazer. Miu is the sharp-tongued childhood friend who knows his deepest secrets. The narrative arc begins with the Awakening, where Fei appears and the first monster attacks. This escalates into a Conflict of Creation, where Yugo must deal with the consequences of his incomplete world. The story expands with the arrival of the Four Heavenly Kings and the antagonist, Emperor Nemesis, who rebel against the creator, Tenka Issetsuna, seeking liberation from the script of the Genesis Tome. This forces Yugo and his embodied characters into a high-stakes action rom-com where finishing his story is the only way to survive.