Su-min and Dong-min find an adorable pet at an antique market in their seaside village and agree to trade some old items from their house for it. When they return home, they discover their grandmother being taken away in an ambulance, and their mother is searching for her shoes. The kids rush back to the shop, but the owner is gone, and they lose their way, falling into another world. There, they encounter four Kokdu figures who mistake them for the dead and guide them to the Sanzu River, the boundary between life and death.