Legend says that centuries ago, a colossal spider ravaged Tokyo. Resistance was futile, as none could quell the havoc wrought upon the city by the beast. Fortunately, one man—the legendary exorcist—stopped the devastation. Using his divine powers, he sealed the monster away and the world has enjoyed peace ever since.

Mizuki Henmi is an adolescent girl who is acquainted with Shu Suzuri, a dull young man who runs an old Japanese bookstore. During a routine visit to his store, Mizuki learns of a mysterious and valuable book that Shu is planning to sell. Unfortunately, the unrestrained Mizuki handles the book roughly, breaking the seal that kept it shut and revealing an odd entity—a cute spider.

Wasurenagumo tells the story of Mizuki, Shu, and their seemingly harmless spider. However, ominous winds begin blowing. While Shu is infatued with his arachnid friend, Mizuki becomes wary of its every move…