"Touhou Bougetsushou ~ Silent Sinner in Blue" is a manga series set in the Touhou Project universe, written and illustrated by ZUN and Aki Eda. The story revolves around an incident in Gensokyo, a land where humans and youkai coexist, involving the moon and its mysterious connection to the Earth. The narrative begins when Reimu Hakurei, the shrine maiden of the Hakurei Shrine, and Marisa Kirisame, a magician, investigate a sudden and unusual lunar phenomenon. The moon appears to be approaching the Earth, threatening to disrupt the balance of Gensokyo.

As the situation unfolds, it becomes clear that the lunar capital, Eientei, and its inhabitants are involved. The enigmatic Eirin Yagokoro, a genius scientist and pharmacist, and her companion Kaguya Houraisan, a moon princess, play central roles in the events. The story delves into the history of the moon and its inhabitants, revealing long-standing tensions between the lunar civilization and Gensokyo. The lunar capital, a technologically advanced society, has maintained a policy of isolation, but the current crisis forces its residents to confront their past and their relationship with Earth.

Reimu and Marisa, along with other prominent characters such as Sakuya Izayoi, the maid of the Scarlet Devil Mansion, and Remilia Scarlet, its vampire mistress, embark on a journey to the moon to uncover the truth and resolve the crisis. The narrative explores themes of conflict, reconciliation, and the consequences of technological advancement, as the characters navigate the complexities of lunar politics and the moral dilemmas posed by their actions. The story also highlights the interplay between tradition and progress, as the characters from Gensokyo, a land steeped in folklore and mysticism, encounter the futuristic and alien world of the moon.

Throughout the series, the characters face numerous challenges, including battles with powerful lunar youkai and the manipulation of time and space. The plot is driven by a combination of action, mystery, and philosophical inquiry, as the characters grapple with the implications of their discoveries and the potential consequences for both Gensokyo and the moon. The resolution of the story hinges on the characters' ability to bridge the gap between two vastly different worlds and find a way to coexist peacefully. The manga concludes with a resolution to the lunar crisis, but leaves lingering questions about the future of the relationship between Gensokyo and the moon.