Keita Amano (voiced by Haruka Tomatsu) is an ordinary 11-year-old boy. One day, Keita was wandering around because of the summer vacation and found a dilapidated gashapon machine under a tree. Keita twisted a gashapon with the only coin he had. In this way, he met Vespa (voiced by Tomokazu Seki), the monster butler. Keita gained the ability to see monsters and summon monsters to assist him. At the same time, he also took on the responsibility of recovering evil spirits. In addition to Keita, these classmates also encountered all kinds of incredible strange things during this summer vacation, including the lively and cheerful but unexpectedly venomous Koreika (voiced by Aya Endo), the huge and scary-looking but gentle Kumashima Gotaro (voiced by Toru Nara), and the materialist Imada Kanji (voiced by Chie Sato), who does not believe in monsters at all.