S4·E4 Hot Spring Party People

Monster Busters is hired to take care of a monster infestation at a normally tranquil hot spring inn in the mountain. They arrive to find that the cause of the commotion isn't monsters but a bunch of partying people, to Fumie's chagrin. But a monster flower emerges from inside the hot spring, and its pollen turns Shinnosuke and his father into party monsters.