"Planetarian: Hoshi no Hito" is a five-episode ONA series that serves as a prequel to the original "Planetarian" visual novel and anime. The story is set in a post-apocalyptic world where humanity has been decimated by decades of war and environmental collapse. The narrative centers on a young man known only as "The Junker," a scavenger who survives by salvaging useful items from the ruins of a once-thriving civilization. During one of his expeditions, he stumbles upon a derelict planetarium in a deserted city. Inside, he encounters Yumemi Hoshino, a humanoid robot designed to serve as a guide for the planetarium.

Yumemi, unaware of the world's devastation, continues to perform her duties as if the planetarium were still operational. Her programming compels her to deliver presentations about the stars and planets, despite the absence of an audience. The Junker, initially skeptical and dismissive of her, gradually becomes intrigued by her unwavering dedication and childlike innocence. Yumemi's persistence in maintaining her role, even in the face of a world that has moved on, creates a poignant contrast to the bleak, decaying environment outside the planetarium.

As the story progresses, the relationship between The Junker and Yumemi evolves. The Junker, hardened by years of survival in a harsh world, begins to soften in her presence, while Yumemi's interactions with him reveal her limited understanding of the world's current state. The narrative explores themes of hope, perseverance, and the enduring human spirit, even in the face of overwhelming despair. The planetarium itself becomes a symbol of humanity's lost dreams and aspirations, with Yumemi as its last remaining guardian.

The series delves into the backstory of the planetarium and Yumemi's creation, providing context for her existence and the world's decline. Flashbacks reveal the efforts of the planetarium's staff to preserve knowledge and inspire future generations, even as society crumbled around them. These moments highlight the contrast between the past's optimism and the present's desolation, underscoring the tragedy of humanity's fall.

The Junker and Yumemi's journey culminates in a bittersweet conclusion that reflects the series' overarching themes. The story remains grounded in its exploration of human resilience and the power of connection, even in a world that seems beyond redemption. Through its concise narrative and focused character interactions, "Planetarian: Hoshi no Hito" offers a contemplative and emotionally resonant experience, rooted in the interplay between its two central characters and the world they inhabit.