Na Jieun strongly believes in destiny and fate. Like how she ends up being a teacher instead of an engineer like she had always want, or had a son at the age of 22. "Fate isn't something we can control. So I just go with it." She always says that. 

Park Chanyeol has life goals and nothing will stop him from chasing his dreams. When he was 10, he predicted that he will graduate medical school at 28, after two years of undergrad education and six years of medical school. "We can alter our future."

Fate brings the two souls together. Fate also brings the future in their life. So when Chanyeol meets Jieun, his life turns upside down.