Inside an abandoned building in a town in Jiangxi province, George Retelas felt something before anyone told him what had happened there.

His hosts guided him inside and told him: two US airmen had sheltered here in 1942, cared for by local villagers until someone who spoke some English could be found to help them escape. When Japanese soldiers swept through, they executed 14 people on the spot and burned the building to the ground.

"The chilling part is they didn't tell me what had happened until I was already inside," Retelas said. "As soon as I walked into that space, I could feel it."

That moment has marked Retelas' journey through a chapter of World War II history that many in the US have never heard about.

It is a story of a wartime alliance of ordinary Chinese civilians who risked and often lost everything to protect strangers from the US who fell from the sky. And it is a story that Retelas, a filmmaker from Silicon Valley, has spent years working to bring to light.

While in college, Retelas discovered a journal and photographs tucked inside a camera bag he had inherited from his grandfather who had served as a US Navy mechanic at Alameda Naval Air Station in California.

One night in early 1942, his grandfather had been rousted from his bed and ordered to help load 220-kilogram bombs onto a mysterious aircraft carrier loaded with Army planes. He did not learn until later that the ship was the USS Hornet, and the bombers were the Doolittle squadron bound for Tokyo.

That discovery sent Retelas down a path of research, then filmmaking, then travel — first across the United States to track down the men who had served alongside his grandfather in Navy Air Group 11, then, eventually, across the Pacific.