The captain of the giant Royal Navy battleship called his officers together to give them a first morsel of one of World War II’s most closely guarded secrets: Prepare yourselves, he said, for “an extremely important task.”

“Speculations abound,” one of the officers wrote in his diary that day -- June 2, 1944.

The secret was D-Day — the June 6, 1944, invasion of Nazi-occupied France with the world's largest-ever sea, land and air armada.

It punctured Adolf Hitler's fearsome “Atlantic Wall" defenses and sped the dictator's downfall 11 months later.

The diary writer was Lam Ping-yu — a Chinese officer who crossed the world with two dozen comrades-in-arms from China to train and serve with Allied forces in Europe.

For 32-year-old Lam, watching the landings in Normandy, France, unfold from aboard the battleship HMS Ramillies proved to be momentous.

His meticulously detailed but long-forgotten diary was rescued by urban explorers from a Hong Kong tenement block before its demolition.

It is bringing his story back to life and shedding light on the participation of Chinese officers in the multinational invasion.

“Saw the army’s landing craft, as numerous as ants, scattered and wriggling all over the sea, moving southwards,” Lam wrote on the evening of June 5, as the invasion fleet steamed across the English Channel.

“Everyone at action stations. We should be able to reach our designated location around 4-5 a.m. tomorrow and initiate bombardment of the French coast,” he wrote.

Sleuthing by history enthusiasts Angus Hui and John Mak in Hong Kong pieced together the story of how Lam found himself aboard the HMS Ramillies and proved vital in verifying the authenticity of the 80-page diary, written in 13,000 Chinese characters.

Hui and Mak have now curated and are touring an exhibition about Lam, his diary and the other Chinese officers — currently on display in the town of Ouistreham in Normandy, France.

One breakthrough was their discovery, confirmed in Hong Kong land records, that the abandoned 9th-floor flat where the diary was found had belonged to one of Lam's brothers.

Another was Hui's unearthing in British archives of a 1944 ship's log from HMS Ramillies.

A May 29 entry recorded that two Chinese officers had come aboard, although it misspelled Lam's surname, reading: “Junior Lieut Le Ping Yu Chinese Navy joined ship."

Lam’s leather-bound black notebook has had a dramatic life, too.

Lost then found, it has now gone missing again.

Hui and Mak say it appears to have been squirreled away somewhere — possibly taken to the U.S. or the U.K. by people who emigrated from Hong Kong — after the explorers riffled through the apartment before the building was demolished.

But Hui got to photograph the diary’s pages before it disappeared, preserving Lam’s account.

Hui and Mak shared Lam's account with his daughter, Sau Ying Lam, who lives in Pittsburgh in the U.S.

She previously knew very little about the WWII experiences of her father, who died in 2000.

Lam was part of a group of more than 20 Chinese naval officers sent during WWII for training in the UK by Chiang Kai-shek.

Chiang led a Nationalist government in China from 1928 to 1949, fighting invasion by Japan and then Mao Zedong's communists, before fleeing to Taiwan with the remnants of his forces when Mao's insurgents took power.

On their long journey from China, the officers passed through Egypt — a photo shows them posing in front of the pyramids in their white uniforms — before joining up with British forces.

In his diary, Lam wrote of a narrow brush with death on D-Day aboard HMS Ramillies, as the battleship's mighty guns were pounding German fortifications to disable them before Allied troops hit the five invasion beaches.

“Three torpedoes were fired at us,” Lam wrote. “We managed to dodge them.”

His daughter marvels at the lucky escape.

“If that torpedo had hit the ship, I wouldn't be alive,” she says.

Through ships' logs, Hui and Mok say they've confirmed that at least 14 of the Chinese officers participated in Operation Neptune, the naval component of the Allied invasion, codenamed Operation Overlord.

Some 7,000 vessels took part.

The Chinese were deployed in pairs, on seven ships, Hui and Mok say.

Some of the Chinese officers, including Lam, also saw action in the Allied invasion of southern France that followed D-Day, in August 1944.

“Action stations at 4 am, traces of the moon still visible, although the horizon is unusually dark,” Lam wrote on Aug. 15.

“Bombardment of the French coast started at 6, Ramillies didn’t open fire until 7."

“The Germans put up such a feeble resistance, one can call it nonexistent.”

France awarded its highest honor, the Légion d’honneur, to the Chinese contingent’s last survivor in 2006.

Huang Tingxin, then 88, dedicated the award to all those who traveled with him from China to Europe, saying “it was a great honor to join the anti-Nazi war,” China’s official Xinhua news agency reported at the time.

Lam’s daughter says their story remains inspirational.

As survivors of the Battle of Normandy disappear, Lam's compelling first-hand account adds another vivid voice to the huge library of recollections that the WWII generation is leaving behind.

It will help ensure that the sacrifices for freedom and the international cooperation that defeated Nazism aren't forgotten.

AP Video shot by John Leicester

Production by Alex Turnbull