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 dreams of becoming the Korean Steve Jobs and possesses a passion and drive to succeed despite her lack of resources. Nam Do-san, the founder of Samsan Tech, is a former math genius who has seen his company's fortunes stagnate. Through a series of misunderstandings, Dal-mi remembers Do-san as her first love and brilliant pen pal from childhood, prompting Do-san to enter the startup world alongside his quirky friends in hopes of turning fiction into reality. As the young entrepreneurs navigate the cutthroat tech industry, they face fierce competition, financial peril, and personal dilemmas. They cross paths with Han Ji-pyeong, a brilliant, sharp-tongued senior investor at SH Venture Capital who feels a deep debt of gratitude to Dal-mi's grandmother. Alongside them is Won In-jae, Dal-mi's estranged sister who chose a privileged life over familial loyalty, only to realize she is merely a pawn in her stepfather's corporate empire. Together, these characters must forge alliances, build innovative solutions, and risk everything for a chance at greatness.