"Isekai Sōzōshu nanode Oshi to Kekkon Shimasu!" centers on a protagonist who is a creator of fictional worlds, known as a "World Creator." This individual possesses the unique ability to design and manifest entire universes, complete with their own rules, inhabitants, and histories. The story begins when the protagonist is summoned to one of the worlds they created, a realm where their creations have taken on lives of their own. In this world, the protagonist encounters a character they designed as their "oshi," or favorite, a figure they had imbued with specific traits and a backstory that made them particularly compelling.

Upon arrival, the protagonist discovers that their "oshi" has developed a deep admiration for them, stemming from the intricate details and care put into their creation. This admiration evolves into a romantic interest, leading to a proposal of marriage. The narrative explores the dynamics of this relationship, as the protagonist navigates the complexities of interacting with a character they once viewed purely as a creation. The story delves into themes of creator-creation relationships, the ethical implications of wielding such power, and the blurred lines between fiction and reality.

The protagonist must also contend with the broader consequences of their abilities. As a World Creator, their presence in the world they designed disrupts the balance, attracting the attention of other powerful entities and factions. These groups either seek to exploit the protagonist's powers or view them as a threat to the world's stability. The protagonist is forced to confront these challenges while maintaining their relationship with their "oshi," who becomes a key ally in navigating the political and magical intricacies of the world.

Throughout the series, the protagonist grapples with their role as both a creator and an active participant in the world. The story examines how their decisions impact the lives of the inhabitants, many of whom are unaware of their origins as fictional constructs. The narrative also explores the emotional and psychological toll of being a creator, as the protagonist struggles with the responsibility of shaping destinies and the moral weight of their actions.

The manga balances romantic elements with action and world-building, creating a layered narrative that appeals to fans of both isekai and romance genres. The protagonist's journey is one of self-discovery, as they learn to reconcile their identity as a creator with their newfound role as a participant in a living, breathing world. The series maintains a focus on the evolving relationship between the protagonist and their "oshi," while also addressing the broader implications of their powers and the consequences of their presence in the world they created.