Season synopsis
Contains spoilers for this season and earlier seasons.
Seo Dal-mi enters Sandbox, a South Korean start-up incubator, hoping to build a company without the wealthy connections enjoyed by her estranged sister In-jae. She believes programmer Nam Do-san wrote the letters that comforted her as a child. Their real author is investor Han Ji-pyeong, who borrowed Do-san’s name at her grandmother’s request. The deception brings Dal-mi into Samsan Tech, the struggling business Do-san runs with his friends. As Dal-mi and Do-san fall for each other, Ji-pyeong becomes both their demanding mentor and a romantic rival.
Samsan develops Noongil, an app intended to help people with impaired sight; Dal-mi’s grandmother Won-deok is losing her vision. A takeover by American company 2STO initially looks like the team’s breakthrough, but its contract retains the programmers and dismisses Dal-mi and designer Sa-ha. Dal-mi breaks up with Do-san and encourages him to leave for America. She seeks work with In-jae instead. When the programmers return to Korea, Dal-mi must recruit them for a self-driving car project, Tarzan, while they are still dealing with their painful separation.
The reunited team faces competition from Morning Group and a ransomware attack linked to rival developers. Dal-mi turns an intended damaging newspaper interview into an opportunity to expose the attack’s source. She and Do-san pursue a major bid and rebuild their relationship. Ji-pyeong accepts her choice, remains involved as an investor and supports a business helping orphans. In-jae ends her legal family connection to her wealthy stepfather and reconnects with Won-deok. An epilogue in 2020 shows Dal-mi and Do-san married, working as Cheongmyeong’s chief executive and chief technology officer. They head to a shareholders’ meeting alongside In-jae and Ji-pyeong.