[10] In June 1592, one month after the sea battle at Sacheon, daimyo Wakisaka Yasuharu arrives at the Japanese naval base in Busan to take charge of the fleet and counter the threat posed by Joseon commander Yi Sun-sin.

From some terrified survivors of the battle, he learns of a new, fully enclosed Joseon battleship which they have dubbed bokkaisen (after a sea monster from Japanese mythology) and mekurabune ("blind ship").

However, that engagement has revealed some critical flaws in its design, such as the fixed dragon head at its bow being prone to getting stuck after making a ramming attack, and its weaker flank armor.

Wakisaka agrees to join forces with the rest of the Japanese navy to ensure a decisive success in their campaign to conquer Joseon and, from there on, Ming China.

After dreaming about his time as a cavalry general against the Jurchen, Yi devises a new strategy which he names "Crane Wing", a crescent-shaped formation designed to trap the enemy from three sides.

However, after identifying and following one of his agents, a troop of Japanese spies discover Yi's base, free their imprisoned countrymen, damage the turtle ships with fire, and escape with their construction plans.

Yi hurries to Suncheon to meet with the vessel's inventor, Na Dae-yong, who is working on an improved version, and declares that he will not use the turtle ships in the next battle, knowing that Wakisaka will discover and exploit their flaws.

After Wakisaka' forces disable them, Yi sends in the improved turtle ship, enabling Eo and Lee to rejoin their lines and complete the Crane Wing.

[3] Writing for Yonhap News, Shim Sun-ah described the film as "a visually spectacular, intriguing prequel of naval epic" with "fast-paced, punchy account of the historical information" and "breathtaking and thrilling scenes.

[43] Choi Tami writing for Maeil Business stated that with clear characters, themes and spectacular naval battle, Hansan has progressed one step compared to The Admiral, and praised the performance of Park Hae-il and other actors.

[44] No Cut News' Choi Young-joo called the naval battle "overwhelming not only visually but also aurally" and praised the director for preserving the realism of the war.

Jo also praised the direction of Kim Han-min who made up for shortcomings of the previous work (The Admiral), and the performance of Park Hae-il and Byun Yo-han.

"[47] Kang Nae-ri in her review for YTN, appreciated the Park Hae-il's portrayal of "a leader who struggles day and night to save Joseon" and Byun Yo-han performance which maintained the tension until the end, and stated that the appearance of the turtle ship "brought excitement and exhilaration to the audience".