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?

Yoo Min-woo's (Song Seung Heon) first love was Soh Eun-hye (Shin Ae). No love was greater than the love between these two...until Eun-hye get's into a car accident. Eun-hye dies...leaving one thing left behind...her heart. Shim Hye-won (Son Yeh Jin) has suffered from a terrible heart disease ever since childhood. Miraculously she finds that she will be obtaining a heart from a donor, Soh Eun-hye.

Suffering from the pain of a heartache, Min-woo goes to Paris to study...the memories of Eun-hye still rest in his heart. When he returns to Korea, fate takes a turn and brings Hye-won and Min-woo together. When the two first meet, Hye-won's heart (Eun-hye's heart), oddly beats faster when she is around Min-woo.

Park Jung-jae (Ryu Jin) is Hye-won's boyfriend. Park Jung-ah (Han Ji Hye) is Jung-jae's sister and she meets Min-woo in Italy and falls for him...

Hye-won doesn't know why or what she feels when she's with Min-woo...and Min-woo feels guilt toward Eun-hye, because the feeling of love rises once again as he continues to see Hye-won...