Gang Cheol (whose name means "iron" in Korean language but he put away his fists and cleaned up his act for the sake of his mother Soon-i, who has dementia. Things are going well for him; he now works as a stevedore at a loading dock and is pursuing a relationship with Soo-ji, a free-spirited woman from Seoul vacationing in the port city. But Soon-i is diagnosed with kidney failure and needs an expensive organ transplant that he can't afford. Knowing that Cheol is desperate for money, local gang leader Sang-gon proposes that Cheol come work for him and his brother Hwi-gon. At first Cheol refuses, but when his debt-ridden best friend Jong-soo gives the deed to Cheol's house to Sang-gon as collateral for a private loan, he's left with no choice but to get dragged back into Busan's criminal underworld. Cheol is ordered to kill a visiting Japanese Yakuza boss whose death will enable Sang-gon's ascension within the Korean branch of the mob organization.