When the family heater runs out of kerosene, the brothers try to force each other out of the kotatsu to get a refill. Upon stopping by a pub on their way from buying some kerosene, Osomatsu, Karamatsu, and Choromatsu find themselves in an underground village populated by a tribe of Dayon people. When the others enter the village through Dayon's mouth, they find their older siblings have grown fond of the village and have started to become Dayon people themselves.