The story, without the spoilers.
Seo Dal-mi dreams of becoming a successful founder in Korea's start-up world, inspired by the memory of letters she once received from a kind pen pal. She joins Sandbox, a fictional start-up hub, and meets Nam Do-san, a brilliant but struggling former maths genius trying to build a company with his friends. Also in her orbit is Han Ji-pyeong, a sharp and successful investor who secretly wrote those childhood letters and has watched Dal-mi from a distance for years. Start-Up is bright, modern and full of energy, with pitch meetings, coding nights, demo days and the thrill of building something from nothing. The love triangle is famous — and divisive — because the second lead is so warm and compelling that many viewers rooted for him. The business story is simplified and the romance sometimes overtakes the start-up plot, but the ambition, friendship and youthful hope are genuinely infectious. The friendships inside Sandbox matter as much as the romance, and every pitch brings the team closer to success or failure.
A little curiosity. A new favourite.
Start with the story, meet the cast, and make room for one more episode.