A nearly forgotten piece of World War II lore is brought back to life in the Chinese epic “Dongji Rescue.”

It’s about the Oct. 1942 sinking of a Japanese transport loaded with British POWs from the capture of Hong Kong, which the Japanese used as an excuse to try and execute their entire “cargo,”

Leave it to the Chinese to remind the world what barbarous bastards the Japanese were during WWII.

But “slow” is the byword in this heroic epic by veteran Chinese TV director Zhenxiang Fei and Guan Hu (“The 800”).

Slow is the submarine (the U.S.S. Grouper) that stalks the unmarked Lisbon Maru, slow enough for free diving Chinese fishermen swimmers to catch up to it to check out the torpedo-firing.

Slowly the Lisbon Maru sinks below the waves with over 1800 British POWs locked in the holds and doomed to die.

And slow is the reaction of the Dongji Island villagers to this horror, who take a lot of time and plenty of extra murderous outrages from the Japanese occupiers before they decide to attempt a rescue via their junk-rigged (sailing) fishing boats.

That doesn’t prevent this formulaic thriller from being moving in its big moments of shared humanity and supreme sacrifice.

The fishermen brothers, marked by their neighbors as having “pirate blood,” live on the other side of the island from the village where a small Japanese garrison runs the show. Both dive and swim like very fast fish, but Ah Bi (Yilong Zhu) is the younger, reckless fisherman. He’s the free diver who spies the U.S. submarine, hears the explosion and fishes a survivor flushed out when the Lisbon Maru is punctured.

Ah Dang (Lei Wu) is the older pragmatist. He’s got dreams of escaping to Shanghai with his fellow outcast girlfriend Ah Hua (Ni Ni). He’s the one to try to shove the injured British survivor (William Franklyn-Miller) under the waves.

“Why borrow trouble,” after all, says Ah Dang (in Chinese with English subtitles)?

The younger sibling won’t hear of it.

We know that Ah Dang will come around to Ah Bi’s humanity and righteousness about the shipload of doomed men that they learn about. It takes a LOT of movie for that to happen.

We know that the “chief” (Haoyu Yang) appointed by the Japanese will have to shake off his appeasing nature, that Ah Hua will have to take a stand. The army deserter/school teacher (Minhao Chen) isn’t just here for drunken comic relief. He has a reckoning with his past coming. And we figure out that Old Wu (Dahong Ni), the protected village sage who once led resistance to the Japanese three years ago when they occupied the island will have to gird himself for one last fight.

“The longer you kneel, the harder it is to rise up again.”

Every obvious thing the script sets us up to expect takes forever to happen.

The middle acts are seasoned with confrontations with the murderous and thin Japanese lieutenant and his trigger-happy garrison, Imperial navy decisions to murder all the Brits rather than transport them to Japan and each of the two “pirate” brothers taking matters and the fight into his own hands in his own way.

Some of the swimming stunts are borderline superhuman, and the sailing and fighting sequences make for delicious spectacle.

It never pays too much to ponder the reasons the Chinese military and its film production companies (check out the Communist Party agitprop logos of the various studios involved) want this particular story to be told. “Heroic” idolizing of Chinese fishermen’s when they being used as pawns for attempted island grabs in the Philippines Sea? Wedge issue “We were FRIENDS to the British back when” used against the Japanese?

American clumsiness in sinking a shipload of Allied POWs played up?

But when this lumbering but intimate combat saga winds its way to a grand, predictable finale, the propaganda and slack pacing aren’t deal breakers. When the chips are down, it isn’t the fact that “They helped us fight the Japanese” that matters. It’s the first law of the sea — when others are in distress, you come to their aid that is the story’s moral compass.

“It doesn’t matter what they look like…A life saved is still a life.”

Rating: unrated, graphic and bloody violence

Cast: Yilong Zhu, Lei Wu, Ni Ni, William Franklyn-Miller, Haoyu Yang, Minghao Chen and Dahong Ni.

Credits: Directed by Zhenxiang Fei and Guan Hu, scripted by Shu Chen, Runnian Dong, Ji Zhang, A Well Go USA release.

Running time: 2:13