Someday is about four different people's story about love.Hana Yamaguchi (Bae Doo-Na) was a successful young cartoonist in Japan. Abandoned by her parents and brought up by her grandma, she was cold and aloof and did not believe in love which to her was only a simple biological response. When the elderly neighbourhood street sweeper died, she discovered the affection between him and her neighbour which lasted 60 years. Hana could not understand this kind of love, so she decided to trace her neighbour's footstep to Korea when she disappeared with her lover's ashes.Go Jin Pyo (Kim Min Joon) was a doctor who was a comic book fan of Hana. He was overjoyed in meeting Hana and believed that he was in love for the first time in his life. He decided to help Hana in fulfilling her wish to look for her neighbour and enlisted the help of a private eye, Lee Seok Man (Lee Jin Wook).Lee was the sole survivor of a car accident whereby his parents and brother were killed. He carried a lot of guilt towards the parents of the other victim and did his best to help out even at the expense of owing money to the loan sharks.Hye Young (Oh Yoon Ah) was a PD of animation projects and a close friend of Jin Pyo. she didn't realize that she was in love with him till Hana appeared.In bringing her neighbour's love story to fruition and published in print, the process changed the lives and outlook on love for these four people.

Ban Ha Jin (Lee Dong Gun), an orphan and high school drop out, is calculating and ambitious. Even though he dropped out of high school and lives with Choi Yoo Kang (Yoon Se Ah) due to extenuating circumstances, he used to be the high school president and hoped to go to college. Through coincidence, he meets Yoon Jae Myung (Lee Jin Wook), the popular former baseball player from high school, at a bar in Switzerland. Ha Jin manipulates a series of events to become Jae Myung's best friend. Also coincidentally, he meets Oh Dan Hui (Kim Hee Sun), the girl he loved and hurt in high school. Oh Dan Hui used to play baseball on the boy's team but now plays softball in college. Her baseball hero is Yoon Jae Myung. All three meet again in Switzerland and again back in Korea. Ha Jin starts working in Jae Myung's father's company and does everything to make the rich chairman like him. Jae Myung starts working as the softball coach of his father's company's team but then gets transferred. He falls in love with Dan Hui who's headstrong and optimistic personality attracts him. He asks Ha Jin for advice, but Ha Jin cannot bear to watch Jae Myung take his true love.

Yoo Eun Ho (Son Yeh Jin), an instructor at a fitness club divorced her husband Lee Dong Gin (Kam Woo Sung) after the death of their new born baby as she blamed him for not being there for her when she needed him most. They remained friends after the divorce and met regularly. Even though both started seeing other people, they still had great feelings towards each other. With the help of YEH's sister Yoo Ji Ho (Lee Ha Na) and LDG's friend, Dr. Gong Jeon pyo (Gong Hyeong Gin), they finally realized that they should get back together.

This drama is about the takeover of the Korean government in 1979 by Chun Doo-Hwan .