Seo Ha Neul (Sung Yuri) and Seo Gun (Gong Yoo) become stepbrother and stepsister when their parents marry each other, and soon come to love each other like real siblings; Ha Neul hero-worships her older brother and Seo Gun will do anything to protect his little sister. But when their parents are suddenly killed, the two are separated: Seo Gun is adopted into a poor family in Australia, while Ha Neul is adopted into a wealthy Korean family. Before leaving, Seo Gun promises Ha Neul that he'll come back for her in a few days. Fiteen years later, Seo Gun returns to Korea - but no longer the innocent boy he once was. He has been forced into a life of crime to pay for the medical needs of his adopted sister Hyo Ju in Australia, and has become a small-time gangster and swindler. Having buried all of his affection for his first step-sister, Ha Neul, his goal in coming back to Korea is to find her and swindle she and her wealthy family out of enough money to pay for Hyo Ju's needs. Ha Neul, meanwhile, has had a very different upbringing, growing up the cherished, sheltered daughter of an overprotective mother and a wealthy chairman. When the two meet again, they can't help but begin to love each other again, but this time, is it the love of siblings, or something more? Haunting and constantly compelling, One Fine Day is one of the more well-known Korean dramas, and assisted its already popular stars well on their way to the superstardom they enjoy now. Highlighted by stunning underwater sequences of the aquarium where Ha Neul works, filmed at Seoul's famous Coex Aquarum, it is a dreamlike and beautiful drama.