In 1974, in the small coastal village of Guncheon, a tight-knit group of haenyeo—Chun-ja, played by Kim Hye-soo, and Jin-sook, played by Yum Jung-ah—struggle to make a living as pollution from a nearby chemical plant ruins their catches. When Jin-sook's father, the boat’s captain Um, played by Choi Jong-won, is approached by a smuggling broker, he is urged to use their fishing boat Maengryong-ho for underwater smuggling. The haenyeo initially hesitate, but they eventually agree to retrieve goods dumped underwater in exchange for money, a choice that brings a sudden and fragile prosperity to the impoverished village.
As the smuggling escalates, Chun-ja convinces the others to participate in a gold-smuggling operation without informing Captain Um. During the mission, a box bursts open, revealing the gold just as customs officers led by Lee Jang-chun arrive. In the ensuing chaos, the ship’s anchor gets stuck, and Jin-sook’s brother Jin-goo is knocked unconscious. Captain Um dies trying to save him, leaving Jin-sook devastated. Chun-ja escapes unnoticed, while the rest of the haenyeo are arrested and jailed, Jin-sook receiving the longest sentence for masterminding the operation. As the members of the smuggling crew are slowly released, they reveal to Jin-sook that Chun-ja escaped and suspect she betrayed them by tipping off the customs about the gold. This suspicion grows stronger as Chun-ja never visits them in prison, and Jin-sook vows revenge once she’s free.
Years later, Chun-ja runs her own smuggling operation in Seoul, but when she encroaches on the turf of a major Seoul smuggler, Kwon Pil-sam, she is captured and tortured. Reconnecting with her old community, Chun-ja and her haenyeo are forced to work with a former crewmember who has become a major smuggler of his own. Chun-ja is crestfallen by the fact that she is no longer the boss she once was. When Chun-ja returns to Guncheon with Kwon to continue their ventures, Jin-sook confronts her. Chun-ja then reveals that she did not betray them to the authorities and explains that she had to hide because she was wanted for stabbing a man who had raped her.
After one of her friends is attacked by a shark, Jin-sook is compelled to collaborate with Chun-ja to help cover medical bills. Jin-sook suspects the customs officers are watching them, so she enlists Go Ok-boon, Go Min-si, who has a history of tax fraud, to lure them as an informant. Ok-boon sneaks into the customs office and steals an informant file, validating Chun-ja’s innocence. Jin-sook apologizes to Chun-ja for misjudging her and plans with her to turn Do-ri and Jang-chun against each other.
As Chun-ja and Kwon plot their moves, Do-ri and his men strike their base unexpectedly. A fierce confrontation ensues, with Kwon’s second-in-command sacrificing himself so his boss and Chun-ja can escape. Kwon dies after a brutal clash with Do-ri and his remaining goons. Chun-ja bargains her life with a shipment of diamonds worth hundreds of millions, while Ok-boon is beaten by Jang-chun for the confession about her deception. Jin-sook exposes Do-ri’s plan to eliminate Jang-chun and seize the diamonds, and Ok-boon’s reveal leads Jang-chun to raid Do-ri’s hideout, finding the informant document in Do-ri’s safe and clearing Chun-ja’s name.
In the final tense sequence, Jang-chun forces Jin-sook and her crew to dive to retrieve the diamond and plans to eliminate them once he secures the diamonds. The haenyeo prove to be formidable underwater, dispatching the thugs with their seasoned diving skills. Do-ri tries to turn the tables with a shotgun, but the crew outmaneuvers him, sending him overboard. Do-ri falls, is chewed by sharks, and meets a grim end as the sharks circle the wounded pirate.
