"Rust-Eater Bisco" is set in a post-apocalyptic Japan where a mysterious rusting disease has ravaged the land, turning everything it touches into decay. The story centers on Bisco Akaboshi, a young "mushroom keeper" who is both feared and misunderstood by society. Mushroom keepers are individuals who cultivate and wield the power of mushrooms, often seen as outlaws due to their association with the spread of the rusting disease. Bisco, however, is determined to prove his innocence and find a cure for the disease, which he believes lies in a legendary mushroom known as the Rust-Eater.

Bisco’s journey begins when he is falsely accused of causing the rusting plague. To clear his name and save his homeland, he sets out on a quest to locate the Rust-Eater, a mythical mushroom said to have the power to reverse the effects of the disease. Along the way, he encounters Milo Nekoyanagi, a kind-hearted but ailing doctor who suffers from the rusting disease himself. Despite their initial differences, the two form an unlikely partnership. Milo joins Bisco in his quest, driven by his own desire to find a cure and protect his younger sister, Pawoo, who also suffers from the disease.

The narrative unfolds as Bisco and Milo traverse a dangerous and decaying world, facing hostile factions, mutated creatures, and the relentless spread of the rusting disease. Their journey takes them through desolate wastelands, crumbling cities, and underground tunnels, where they uncover secrets about the origins of the rusting plague and the true nature of the Rust-Eater. Along the way, they encounter allies and adversaries, including the militaristic Imihama Prefecture, which seeks to control the Rust-Eater for its own purposes, and the enigmatic Actagawa, a giant crab that becomes a loyal companion to Bisco.

As the story progresses, Bisco and Milo’s bond deepens, and their shared goal of saving their world from ruin drives them forward. The series explores themes of survival, trust, and the resilience of humanity in the face of environmental collapse. The dynamic between Bisco’s rugged determination and Milo’s compassionate idealism creates a compelling narrative tension, while the richly imagined post-apocalyptic setting adds depth to the story. "Rust-Eater Bisco" combines action, adventure, and emotional stakes, offering a unique take on the quest for redemption and hope in a decaying world.