The film mainly follows the famous 1597 Battle of Myeongryang during the Japanese invasion of Korea 1592-1598, where the iconic Joseon admiral Yi Sun-sin managed to destroy a total of 133 Japanese warships with only 13 ships remaining in his command. The battle, which took place in the Myeongryang Strait off the southwest coast of the Korean Peninsula, is considered one of the greatest victories of Yi.

Gang-du is a dim-witted man working at his father's tiny snack bar near the Han River. Following the dumping of gallons of toxic waste in the river, a giant mutated squid-like appears and begins attacking the populace. Gang-du's daughter Hyun-seo is snatched up by the creature and he and his family are taken into custody by the military, who fear a virus spread by the creature. In detention, Gang-du receives a phone call from Hyun-seo, who is not dead, merely trapped in a sewer. With his family to assist him, he sets off to find Hyun-seo.

1986 Gyunggi Province. The body of a young woman is found brutally raped and murdered. Two months later, a series of rapes and murders commences under similar circumstances. And in a country that had never known such crimes, the dark whispers about a serial murderer grow louder. A special task force is set up in the area, with two local detectives Park Doo-Man and Jo Young-Goo joined by a detective from Seoul who requested to be assigned to the case.