Jang Eun-ha, [Park So-dam], is a former North Korean defector turned special delivery clerk in Busan who travels to Seoul to pick up a former baseball player, Kim Doo-sik, [Song Sae-byuk], who is involved in an illegal gambling trade. At the pick-up point, Eun-ha meets Doo-sik's son Kim Seo-won, [Jung Hyeon-jun], and the pair quickly attract the attention of Kyeong-pil, a corrupt police officer and the mastermind behind the gambling operation. Kyeong-pil is after Seo-won because he wants to retrieve a security key to a bank account containing US$30 million, a fortune that Seo-won holds in his possession, setting off a dangerous game where trust is scarce and every move could be fatal.
Doo-sik sacrifices himself while trying to protect Seo-won, and in the ensuing chaos, Kyeong-pil frames Eun-ha for Doo-sik's death and for Seo-won's abduction. Kyeong-pil's henchmen launch another attempt to seize Seo-won, but Eun-ha exacts a hard-won victory in an intense chase, subduing the attackers even as the stakes grow higher. The mastermind's machinations ripple outward, dragging many others into the fray and threatening to unravel everything Eun-ha has left.
Back in Busan, Eun-ha narrates the shocking events to her boss, who devises a plan to forge passports and send Seo-won abroad. While they work on the fake documents, Kyeong-pil ambushes them in a brutal confrontation that leaves Eun-ha's boss dead and forces Eun-ha to respond with all the force she has. She engages Kyeong-pil and his men in a desperate confrontation; at one point, Kyeong-pil holds Seo-won at gunpoint, casts him into the sea, and fires toward Eun-ha, but she fights back with grit and resolve, stabbing Kyeong-pil and leading to a fatal downfall for him as they both drown in the cold water. Seo-won is saved, but Eun-ha disappears under the cover of a staged death, slipping away from the pursuing authorities.
Days turn into a quiet, fragile life for Seo-won as he is taken in by an orphanage and starts a hopeful routine at school, gradually finding a sense of safety and normalcy. When he and Eun-ha finally cross paths again, they drive toward the orphanage together, but a new message about a client arrives for Eun-ha, prompting her to tell Seo-won that they should head to a pick-up point—an ominous signal that she has not truly left behind her life as a special delivery clerk, and that the danger she embodies might be far from over.