One day at the end of summer vacation, the monster hunter agency receives a telephone call. "Many swimmers went missing mysteriously. Please examine what is going on.". They get happy because they didn't get to go anywhere during summer vacation. Mikoto, Takeru and Sui head for that isolated island to investigate if the events are related to monsters. During their stay on the island, they reside in the house of the islands owner. They enjoy swimming and hunt any monsters they encounter, but their investigation reveals the presence of the big and stronger monster behind the scenes....