After falling in love as teens, two young adults drift apart — only to reunite 10 years later and confront their heartbreak, growth and new challenges.

Ace NIS agent Jung Hae-seong is demoted after a mission failure. He goes undercover at a high school to locate King Gojong's missing gold, meeting his teacher Oh Soo-ah, who sees him as her lost first love, stirring old heartbreak.

Kwon Si-Hyun bets his life on a dangerous game to seduce Eun Tae-Hee. His target is 20-years-old and believes people that are swayed by love are pathetic. After Eun Tae-Hee meets Kwon Si-Hyun, her view on love changes. Based on 18th century French novel "Les Liaisons dangereuses" by Pierre Choderlos de Laclos.