"Planetarian" is a five-episode anime series adapted from a visual novel by Key. Set in a post-apocalyptic world, the story unfolds in a desolate city ravaged by war and constant rain. The narrative centers on a nameless protagonist, referred to as the "Junker," a scavenger who stumbles upon an abandoned planetarium while searching for supplies. Inside, he encounters Yumemi Hoshino, a humanoid robot who serves as the planetarium's guide. Despite the planetarium being in ruins and devoid of visitors for decades, Yumemi continues to perform her duties, unaware of the world's collapse.

Yumemi is programmed to provide planetarium shows and interact with guests, but her outdated systems and lack of understanding about the outside world create a poignant contrast to the bleak reality. The Junker, initially indifferent and focused on survival, gradually becomes drawn into Yumemi's unwavering optimism and dedication. As they interact, Yumemi expresses her desire to repair the planetarium's projector, hoping to fulfill her purpose of entertaining visitors. The Junker reluctantly agrees to help, and their efforts to restore the projector form the core of the story.

Throughout the series, the relationship between the Junker and Yumemi evolves, highlighting themes of humanity, perseverance, and the remnants of a lost civilization. Yumemi's childlike innocence and unwavering belief in her mission stand in stark contrast to the Junker's hardened, pragmatic outlook. The narrative explores the tension between hope and despair, as Yumemi's idealism clashes with the harsh realities of the world outside the planetarium.

The climax occurs when the projector is finally repaired, and Yumemi delivers a heartfelt planetarium show for the Junker, her first and only audience. However, their brief moment of joy is interrupted by the arrival of hostile forces, forcing the Junker to confront the dangers of the outside world. The series concludes with a bittersweet resolution, emphasizing the fragility of hope in a broken world and the enduring impact of human connections, even in the face of inevitable loss. The story of "Planetarian" is a contemplative exploration of memory, purpose, and the enduring spirit of humanity, encapsulated in the interactions between a lone scavenger and a steadfast robot.