According to legend, Tokyo was once terrorized by a colossal spider that no one could conquer. However, a legendary exorcist managed to use his divine powers to seal the monster away, bringing peace to the city for generations. In the present day, Mizuki Henmi is a teenager who frequents a quaint Japanese bookstore run by the unassuming Shu Suzuri. On one of her visits, Mizuki learns about a valuable and mysterious book that Shu intends to sell. Despite his warnings to be careful, Mizuki damages the seal on the book and unknowingly releases a peculiar entity—a cute spider. Wasurenagumo follows the story of Mizuki, Shu, and their seemingly harmless spider. As the trio's bond grows stronger, foreboding winds start to blow. While Shu is enamored with their new arachnid companion, Mizuki begins to feel uneasy about its every move...