The concept of an arranged marriage is often difficult to accept, especially amongst less traditional adolescents today. Sweet 18 (Nang Rang 18 Seh) covers this very premise and the story begins when Kwon Hyuk-joon's grandfather, the head of the traditional Kwon family agrees to an arranged marriage between his grandson (Lee Dong Gun) and Yoon Jung-sook (Han Ji Hye) when Yoon Jung-sook is born. 18 years later, the two families lose contact because soon after the contract is signed, Jung-sook's family encounters financial issues and is forced to run away.
Meanwhile, Jung-sook has grown up to be a rebellious and strong-willed girl who just wants to live her life as she sees fit. With little aspirations, she is the direct foil of her achievement-bound counterpart, Hyuk-joon. When the two meet each other, they clash horribly; Jung-sook thinks her fiancee is nothing but boring, and Hyuk-joon believes Jung-sook to be nothing but an immature child. The rest of the series follows their life living together as a couple, and the obstacles they are faced with. When Hyuk-joon's first love reappears and wants to claim Hyuk-joon for herself (despite the fact that he's married), who will Hyuk-joon choose? His first love Ga-young, or immature spunky Jung-sook?