Former national swimmer Cheon-soo dreams of travelling to Palau, and to fund his trip he works as a swimming instructor by day while gambling at night. However, after losing a game of poker he finds himself heavily in debt and in trouble with the loan sharks. Kang, the head of a local drugs syndicate, offers to pay off Cheon-soo's debts; in return, Cheon-soo must work as a "marine boy", a Mule (smuggling) who smuggles drugs across the open waters.