Set in South Korea’s fictional Silicon Valley called Sandbox, Start-Up tells the story of people in the world of startup companies. Seo Dal-mi (Bae Suzy) is a bright and ambitious young woman who dreams of becoming Korea’s Steve Jobs. Dal-mi doesn’t have a fancy background but she’s passionate about her work. She has bright energy and is a person of great vitality, having experience in a wide range of part-time jobs. Nam Do-san (Nam Joo-hyuk), is the founder of Samsan Tech. A ‘math genius’ as a young boy, Do-san was once the pride of his family but became their shame now, as his business has been going down for the past two years. He finds out that Dal-mi mistakenly remembers him as her first love, so he decides to work his way up in hopes of turning that misunderstanding into reality.