In 1597, Korean admiral Yi Sun-sin, who had successfully managed the construction of a formidable fleet of warships, was stripped of his rank, tortured, and nearly executed, with naval command transferred to his rival, Won Gyun, due to political machinations.
Won Gyun promptly loses much of the fleet in several disastrous engagements with the Japanese, leaving the Koreans heavily outnumbered.
Yi is hurriedly reinstated, but by this point he commands just 13 ships against a fleet of at least ten times that size. Many of his ships are crewed by survivors of the previous battles, and fear a return to combat.
Yi carefully selects a narrow, shallow strait for his "final stand" limiting the size and number of Japanese ships that can attack simultaneously.
Yi's flagship initially engages the Japanese attackers alone, due to the other ships' hesitancy. As it repels one ship after another, like "a castle in the sea", the other ships eventually join, and the Japanese fleet is repelled.
The Japanese lose at least 30 ships. The Korean fleet loses none.