Dahyun Kim, the first girl to be born in the kim family for centuries, received the family heirlum, a pocket watch. But she doesn't know that there's something special about the watch, it's not just a simple heirlum. It has a lost city inside it, where the kims get the woman they will marry at the age of 23. what will happen to Dahyun if she accidentally went inside the pocket watch, at the age of 23 and met the lost city's princess, Sana Minatozaki.