Admiral Yi Sun-sin of Korea began his legendary career with a series of disasters. Fate (and corrupt officials) conspired against him to have him repeatedly knocked down from the success he had earned, often because his insistence on strict military codes and refusal to ignore corruption made enemies of his fellow officers. Even when his superior had him tortured and blamed after a loss to the Jurchen raiders from the north, Yi perservered. Stripped of his rank and now reduced to a common enlisted man, Yi nevertheless served Korea with distinction. Meanwhile his childhood friend, Ryu Seong-ryong, had risen to become the prime minister of Korea. Ryu recognized the threat of war from Japan looming on the horizon, so when Yi asked to retire in 1588, Ryu convinced him to stay.

Japan invaded Korea after a series of long civil wars that had finally culminated in Toyotomi Hideyoshi leading a unified Japanese army. Their martial society had trained extensively with weapons like the arquebus, early guns, and the civil war had given them tremendous experience with siegecraft. By contrast, Korea had not been at war for hundreds of years: they were mostly troubled by raiders from without and corrupt government officials from within. However, their unique situation meant that they had great cannons for fighting off pirates and secure if minimal hill-top forts. As a tributary ally of the Chinese, the Koreans were startled and confused when Japan asked permission to march through their territory and make war on China. Many officials thought the Japanese were only bluffing, but Ryu Seong-ryong recognized the threat and made sure his friend Yi was moved to naval service to help defend the country. Yi trained his men and commissioned a new ironside ship design called the Turtle Ship. Unfortunately, other commanders did not take the threat seriously: even when Japanese ships appeared on the horizon, the southern commander convinced himself it was a trade fleet and took no action while the ships docked, then unloaded their soldiers onto Korean soil. The well-trained Japanese army crushed the Korean army and quickly advanced to Seoul. Meanwhile, Yi organized his small fleet of warships and launched quick strikes against the Japanese navy, catching them off-guard and on-patrol. He destroyed 43 enemy ships without losing a single one of his own, and was promoted accordingly to become the new Southern Commander of the Korean navy.

While Yi found success at sea, the Korean land army suffered terrible losses. Yi Il, the man who once accused Yi of negligence, lost one battle after another, until finally the regular forces were annhilated at Chungju. The Joseon court that ruled to Korea fled to Pyongyang, on the verge of being pushed out of their own country. But that same day, Admiral Yi tore through a Japanese fleet at Okpo. He moved on to Sacheon, where he baited the Japanese commander into a trap and debuted his turtle ship. The unstoppable turtle ship carried the day, so he used this tactic again and again he destroyed a Japanese fleet while suffering no losses of his own. Finally, Hideyoshi ordered his naval commanders to take Jeolla, Yi's headquarters. Sadly for him, his general Wakisaka Yasaharu grew too eager and engaged Yi without backup at Gyeonnaeryang Strait, only to find himself lured into an even more sophisticated version of Yi's bait-and-retreat strategy: a "Crane's Wing" of ships that collapsed on the overextended target from all sides. In one of the largest naval battles in history, Yi scored a decisive win and again didn't lose a single ship. He headed to Angolpo to attack Hideyoshi's two remaining generals and seal his victory, but they refused to be baited. He had to settle for a long range exchange of cannon fire, which worked at the cost of many injuries to his own men. In the end, he destroyed all but a few Japanese ships, and those he only spared to give the Japanese some means to escape and stop raiding in Korea. But he had accomplished his goal: Hideyoshi ordered a halt to all naval operations except guarding Busan, and without this control of the sea, Japan could not re-supply their troops nor hope to resume the assault that would have finally pushed Korea's leaders out of Korea.

Yi's success had forced the Japanese to give up offensive naval operations, but their huge fleet remained entrenched in Busan harbor. While Yi pinned them down, reinforcements from the Chinese army had finally arrived and helped the Korean army take back the country on land. Yi petitioned for marines to take Busan back from the Japanese, but his requests were ignored. Instead, he focused on making his base on Hansando self-sufficient: he promised protection to refugees in exchange for them working the island, building his equipment, and even researching military technology. But a truce was called with Japan, one that dragged on for years until Hideyoshi broke it by ordering a second invasion. An informant brought word of secret, unprotected Japanese fleet movements, but Yi recognized it as a trap and refused to go. However, his friend Ryu's enemies at court seized on this as an opportunity to put Yi on trial for treason. They demoted him again, and gave his fleet to Won Kyon. Won Kyon fell into the trap Yi had refused, and a coordinated surprise attack from the Japanese resulted in the destruction of all but 12 ships. Yi was quickly re-instated, but ordered to disband the navy. He refused, and planned his counterattack carefully: he would fight at Myeongnyang Strait, where he hoped the natural currents would do what his numbers could not. His plan worked: the reversing tide caught the Japanese by surprise and flung their ships against each other right as he pressed the attack. With 13 ships versus 133, he once again drove back Japan with zero losses to his own navy. Word of his success brought other ships out of hiding and convinced the Chinese navy to ally with him at last.

After his success at Myeongnyang, Yi began rebuilding the Korean navy and strengthening his partnership with the Chinese. But then, Toyotomi Hideyoshi died. Japan's new leaders had no interest in continuing the war, but although they sued for peace, Korea now held the upper hand and was determined to punish the people who had committed so many massacres against their people. Yi and the Chinese fleet bottled up the Japanese at the fort in Suncheon. When the Japanese called for reinforcements, Yi interrupted them in Noryang Strait. Again they were outnumbered, 500 to 150, but the Chinese commander did not yet understand Yi's long range style of warfare and immediately closed for close combat. Yi ordered his flagship to rescue their allies, and as soon as the Japanese recognized him, they focused fire on him. This allowed the Chinese, suddenly forgotten, to fire freely on them. The Japanese realized their error and tried to flee, but Yi would have none of it. Beating the war drum himself, he urged his ships to chase the Japanese - to punish them for all the slaughter they brought to Korea. It was then that he was struck by a fatal gunshot. Before he died, he ordered his son and nephew to command the battle for him. They dressed in his armor to hide his death from the troops and continued beating the dream. Together, they carried the day - only for Yi's tragic death to be revealed at the moment of victory. But although Yi did not live to see it, 300 Japanese ships were captured and destroyed that day and the rest of their invading force was rounded up soon after. For his tireless service, his brilliant leadership, and his unwavering devotion to Korea, Yi was given the posthumous title of Chungmugong, the Martial Lord of Loyalty.