"Diasta of Ultima Thule" is a manga series set in a post-apocalyptic world where humanity struggles to survive in the remnants of a once-thriving civilization. The story unfolds in Ultima Thule, a desolate and frozen wasteland that serves as the last refuge for humans. The narrative centers on Diasta, a young woman who possesses a mysterious power tied to the ancient technology of the old world. This power, known as the "Thule Code," allows her to manipulate and control the remnants of advanced machinery scattered across the land.

Diasta is accompanied by a group of survivors, each with their own motivations and backstories. Among them is Ragnar, a hardened warrior with a deep sense of duty, and Freyja, a resourceful scavenger who seeks to uncover the secrets of the old world. Together, they navigate the treacherous landscape, facing threats from both the environment and hostile factions vying for control of the remaining resources. The group's journey is driven by the need to find a rumored sanctuary known as the "Eternal City," a place said to hold the key to humanity's survival.

As the story progresses, Diasta's connection to the Thule Code becomes increasingly significant. Flashbacks and fragmented memories reveal that she may have been part of an experiment conducted by the ancient civilization before its collapse. This revelation raises questions about her true identity and the origins of her powers. The group encounters remnants of the old world, including autonomous machines and cryptic messages left behind by the ancients, which hint at the catastrophic events that led to the current state of the world.

Conflict arises not only from external threats but also from internal tensions within the group. Ragnar's pragmatic approach often clashes with Freyja's idealism, while Diasta struggles with the moral implications of wielding her power. The narrative explores themes of survival, sacrifice, and the consequences of humanity's reliance on technology. As the characters delve deeper into the mysteries of Ultima Thule, they uncover truths that challenge their understanding of the past and their hopes for the future.

The manga balances action, mystery, and character development, with a focus on the interplay between the protagonists and the harsh world they inhabit. The story builds toward a climactic confrontation with a powerful faction seeking to exploit the Thule Code for their own gain, forcing Diasta and her companions to make difficult choices that will determine the fate of humanity.