According to legend, a huge spider destroyed Tokyo centuries ago. Resistance was futile, as no one could stop the beast from wreaking destruction on the city. Fortunately, one man—the famed exorcist—was able to stop the carnage. He sealed the beast away with his almighty abilities, and the world has been at peace ever since. Mizuki Henmi is a young adolescent who meets Shu Suzuri, a drab young guy who owns an old Japanese bookstore. Mizuki learns of a rare and precious book that Shu is going to sell during a routine visit to his store. Unfortunately, the unconstrained Mizuki roughs up the book, breaking the seal that kept it closed and unveiling an unusual entity—a cute spider. Wasurenagumo follows the story of Mizuki and Shu, a couple who appear to be in love.