Legend has it that centuries ago, a gigantic spider devastated Tokyo. Resistance proved futile, as no one could stop the chaos the beast unleashed upon the city. Fortunately, a single man—the legendary exorcist—halted the destruction. He used his divine powers to seal the monster away, and the world has known peace ever since. Mizuki Henmi is a teenage girl who knows Shu Suzuri, a rather dull young man who runs an old Japanese bookstore. During a regular visit to his shop, Mizuki discovers a mysterious and valuable book that Shu intends to sell. Unfortunately, the impulsive Mizuki handles the book carelessly, breaking the seal that kept it closed and revealing a strange entity—a cute spider. Wasurenagumo follows the story of Mizuki, Shu, and their seemingly harmless spider. However, ominous winds begin to blow. While Shu is infatuated with his arachnid companion, Mizuki grows wary of its every move…