One Fine Day

Seo Gun and Seo Ha-neul are not related by blood, but they become step-siblings when Gun's father marries Ha-neul's mother. The marriage is short-lived: both parents die, and the two children are sent to an orphanage, where they are separated after being adopted into different families. Before they part, they promise each other they will meet again one day.

Fifteen years later, Gun (Gong Yoo) is getting by in Sydney, Australia, working the docks and running fake-marriage schemes with his old friend Kyung-taek, whom he lives with along with Kyung-taek's daughter Hyo-joo (Lee Yeon-hee), who has quietly loved Gun since they were children. Ha-neul (Sung Yu-ri) was adopted into the wealthy Park family and now goes by Park Hae-won, working as an aquarist, though her life inside that family is far from easy. When Gun's friend Sung-chan finds Ha-neul's whereabouts, Gun heads back to Korea planning to con his estranged sister out of money.

Posing as her brother to get close to her, Gun cannot go through with the swindle and starts protecting Ha-neul instead, from her controlling adoptive family and from Park Tae-won, a relative who takes an unwelcome interest in her. As the two spend more time together, what should be sibling affection turns into something neither of them expected, complicated further by Ha-neul's aquarium colleague Kang Dong-ha (Namkoong Min), who also falls for her. The story turns when Gun and Ha-neul learn that her adoptive father, Park Jin-kwon, was responsible for the accident that killed Gun's parents years earlier.

Once the truth about their non-blood relationship and Park Jin-kwon's role in their parents' deaths comes out, Gun and Ha-neul are finally free to choose each other, while Hyo-joo comes to terms with her own unrequited feelings. The series ends with Gun and Ha-neul reunited at the aquarium where she works, years after the promise they made as children.