“Noryang: Deadly Sea,” is a slow-start/big finish epic about Korea’s years-long 16th century conflict with Japan, the Imjin War.

Director Kim Han-min’s CGI-aided sea spectable is the third film in a trilogy about that struggle, following “The Admiral: Roaring Currents,” and “Hansan: Rising Dragon.” This is top-down history that evolves, over the course of this long final film, into combat at its most personal.

We view the conflict from on high, through the Byzantine intrigues of the Japanese court, an uneasy Korean (Joseon) alliance with the Chinese Ming Dynasty. We see the sea battle from the bridge as the commanders test each other over a night-long climactic action in rowed war galleys, which by the time of this conflict — the 1590s — were being superceded by warships powered exclusively by sail, in Europe, at least.

The recently unified Japanese, here referred to by their ancient name, the Wae, invaded Korea with an eye towards marching directly on the Ming on the Chinese mainland. That didn’t work out, and the unifying Japanese warlord, “chief counselor” realizes this on his deathbed.

“Withdraw” their last remaining garrisons on the south of the peninsula is among his final words.

But with the passing of the “unifier,” Japan has both an inherited ruler, and factions vying for power. That “withdrawal,” it is implied, would change the status quo. Some want to fight on, some want to negotiate an exit to preserve land and sea forces, and depending on which day you’re asking them, the Chinese and the Joseon Koreans are willing to listen or are more interested in playing the angles.

That scheming and negotiating eats up the first hour of the film. And for all that, little is made as clear as that last paragraph’s summary.

Putting fleets to sea, plotting strategy, setting traps and springing them on each other takes up most of the latter two acts of “Deadly Sea,” which is a good thing. The swirl of characters and the chaos of combat is where this historical thriller gets its sea legs.

Our three antoganists are the celebrated Korean Admiral Yi Soon Shin (Kim Yoon-seok), the Japanese fleet commander Shimazu (Baek Yoon-Seok) and the Ming military leader Chen Lin (Jeong Jae-yeong).

Complicating matters, Yi Soon Shin lost his son to a Wae massacre, and everybody is sure he and his surviving family want “revenge.” Shimazu is depicted as cagey, determined not just to arrange an evacuation of Japanese invaders, but to “win the war” at sea, gambling that an allied Japanese fleet will join the action in time to impact the outcome.

And Chen Lin has decided that the war is over, the Wae/Japanese have lost, and that no more bloodshed will be necessary. A mere “show of force” will do.

The battle to come will be fought with gigantic armored “turtle ships,” and smaller galleys, with cannons and rockets and muskets and bombs and archery and swordfighting. And drums. There was lots of drumming in rowing galleys, if you remember your “Ben-hur.”

Kim Han-min lets us see maps and war planning, and uses establishing overhead (CGI) shots of fleets in action to recreate the flow of battle in the inky darkness, and then zeroes in on commanders waving swords and shouting “Charge” (in Korean, mostly, with English subtitles) or “ROTATE” or “Withdraw!”

We only glimpse the rowers once, but there’s fierce deck combat — fires and explosions and hailstorms of musketballs or arrows — and all that’s before the order “Prepare to BOARD the enemy!” is barked.

The action sequences make the movie, but it’s also interesting for a non-Asian viewer to take in this picture’s attitudes about its source culture and its neighboring rivals. The Koreans are intrepid, brave and sage, recognizing that the Japanese cannot be trusted and that they will never surrender. The Japanese are cunning, stoic and arrogant, with the battle perhaps avoidable if they’d just “apologize” for their first and not last invasion of Korea.

The Chinese? They’re cagily playing the odds, willing to listen to bribes, honoring their alliance with Korea only up to a point. No, the Koreans can’t trust them, either. Neither can the Japanese.

Performances tend to get lost on canvases this broad, but the leads register, even if the heroic or cowardly offspring and members of the lower ranks get short shrift.

And that first hour and eight minutes of backstory, debates and scantily-detailed planning burdens “Noryang” to an almost unforgivable degree.

I don’t recall whether I reviewed the first film in this trilogy when it came out. But remembering the “turtle ships/secret weapon” nature of “Hansan,” this film’s shifting fortunes, surprise turnabouts and sheer spectacle make it a fine finale, not so much an argument for Korean nationalism as a call to arms to resist the Chinese and Japanese versions of it in a roiled world whose politics are in flux.

Rating: unrated, bloody violence

Cast: Kim Yoon-seok, Jeong Jae-yeong, Baek Yoon-Seok, Yeo Jin-goo, many others

Credits: Scripted and directed by Kim Han-min, scripted by A Well Go USA release.

Running time: 2:32