Tianshui (Kim Kang-woo), a young man who is familiar with water, lost his money in gambling and has fallen to teaching people to swim and cleaning in the swimming pool. He was beaten up by a group of gangsters. Fortunately, another group of people appeared in time to save his life. After they tested his water ability and took him into a small house, Tianshui met the drug lord President Jiang (Cho Jae-hyun), who was asked to do a little favor for them to write off the loan sharks owed. It turned out that the drug trafficking organization was planning a transnational drug trafficking operation. They were looking for a "sea boy" to help them transport the latest drugs across the sea to Japan, and Tianshui became their latest target. At this time, he had already been spotted by the police squad leader Jin (Li Yuanyu), who arrested Tianshui in the name of Tianshui's participation in gambling and asked him to go undercover into the drug trafficking organization. In the process of contacting President Jiang, Tianshui accidentally met President Jiang's daughter Youli (Park Shi-yeon). Since then, Tianshui's life has fallen into a strange journey full of crises.