Former national swimmer Cheon Soo dreams of traveling 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 drug syndicate, offers to pay off Cheon Soo's debts; in return, Cheon Soo must work as a "marine boy", a mule who smuggles drugs across the open waters.
Knowing the danger he faces, Cheon Soo tries to make a run for it, but he is arrested at the airport by police detective Kim Gae Ko. Detective Kim is intent on capturing Kang and faced with no other choice, Cheon Soo agrees to work as a spy on his behalf. His situation becomes further complicated when he falls for Yu Ri, a jazz singer under Kang's charge and the daughter of Kang's best friend. But Yu Ri suspects Kang of killing her father, and she and Cheon Soo plot to take the drug money for themselves.
