Following the fourth film, the story follows 11-year-old Keisuke Amano, the younger brother of Natsume from the original Youkai Watch series. Initially skeptical of the supernatural, Keisuke grows curious when he discovers his sister's late-night activities. To his surprise, Natsume is involved in exorcising malevolent Youkai with her friends Touma and Akinori, operating a Yo-Kai detective agency. Upon learning the truth, Keisuke joins their quest, assisting in unraveling Youkai mysteries and battling dark forces to safeguard the city's tranquility.