About half of Hansan is a naval battle. The second half. The first half is a combination history lesson, period espionage and turgid war thriller, and naval warfare theory symposium. The film’s about Admiral Yi Sun-shin, who kicked the invading Japanese navy’s ass in the sixteenth century. Despite being in command, lots of folks questioned Yi, and then he also was trying new tactics and types of warships. Park Hae-il plays Yi. He’s almost indistinguishable from a wax sculpture; Yi was a pensive, reserved fellow, but Park plays him without any personality whatsoever. Not because Park’s bad, but because director and co-writer Kim Han-min doesn’t do character. Hansan’s utterly absent memorable characters, which is something else for a war movie.

It’s also fine because Hansan is a history lesson. There’s a compelling but narratively problematic prologue with Japanese admiral and general dick Byun Yo-Han inspecting a destroyed warship. The Korean navy has some kind of “turtle ship” with a Dragon head on it, which terrified the ship’s crew as it destroyed the vessel. Now, there have been numerous movies about mystery vessels; at least three James Bonds and maybe a Godzilla. Except there’s no mystery. It’s just Park’s latest idea, though he doesn’t like the dragon head.

Kim and co-writer Yun Hong-gi pull back on the narrative distance so incredibly far their characters lose all perspective. Despite Hansan’s first hour being about Byun wondering what Park’s going to do, while Byun’s allies give him shit and Park’s allies give him shit, and they both try to spy on one another, no one ever learns anything in the film. It’s a history movie with the cause and effect removed.

It also doesn’t matter because the second half is a thrilling naval war movie about the application of firepower on sea-going vessels. Hansan shows its hand in the first half; Park drills the Korean navy with the tactics he’s going to use in the second half. The movie shows off the shark first thing (relatively) but still gets plenty of mileage out of it in the battle. There are some surprises, of course, which unfold the same way as the rest of the film’s reveals. A character is alone, remembering a plot twist a few scenes before, completely changing the nature of their subplot. The film does it at least three times, possibly four, saving a major—but not—reveal for the finale.

But it all still works because Kim pulls off the sea battle. There are some land battles too, which he does okay with, but clearly, the thought went into the warships, and it shows.

The best performance is easily Byun, who gets to relish in unrestrained villainy while almost everyone else has to show some decorum. Kim Sung-kyu is good as an enemy prisoner who coincidentally encountered Park in the flashback. Park Ji-Jean has a fun part as the shipbuilder. Park’s okay; the movie doesn’t ask him to do anything, just stand there. Admittedly, there aren’t many options when you’re just supposed to be watching some quiet thinking guy quietly think.

The technicals are all solid. Han Hyun-gun and Lee Gang-here’s editing is a little impatient in parts—there’s a three or four-minute history lesson montage after the prologue, and it’s too hurried. After threatening dozens of characters, Hansan boils down to like six people before the sea battle. Kim and Yun get way too complicated. Once it settles into the espionage subplot, with actual players, it works much better.

But, again, doesn’t matter so long as the sea battle pays off. The movie starts promising a great sea battle, then delivers it. Along the way, there’s some good filmmaking, decent acting, and compelling history, if not character drama.

Hansan’s a qualified, impressive success.