Helen's eclipse was so complete that when the actor and writer Tim Considine read about Edgar's story in the mid-1970s and wanted to make a feature film about him (Edgar died in 1972), his first reaction was to contact the "Mrs Snow living in New York".

"I called and we discussed the issue," Considine said. "Mrs. Snow wanted a particularly high level of control over the proposed film, so I dropped the idea and kind of forgot about it."

That was until one day when Considine's then-girlfriend, who worked at Warner Brothers Studio, presented him with some unpublished diaries and manuscripts that had landed on the desk of the studio chairman.

"They were penned by Helen," Considine said. "Only then did I realize that I had been talking to the second wife, that there was another Mrs Snow who had gone through it all with Edgar. I didn't know that; no one in America knew."

He immediately found Helen's address and called and persuaded her that they should meet at her home, where she had led the life of a hermit since 1941.

"I just was charmed by her. ... We talked all day long and I eventually missed the last train and stayed for the night," said Considine, to whom Helen later made frequent phone calls and sent "volumes of letters", in which "she told and retold stories from the '30s".

One of those phone calls was made in 1977, when Helen told Considine that "Huang Hua was coming to New York and there will be a reception for him, but I'm not going".

"You must go," Considine remembers himself saying.

So at the New York reception one month later, Considine found himself sitting on a foot stool between Helen and Huang, who had shut themselves in a quiet room to have a proper talk where they sat on "two big overstuffed chairs". Considine tentatively proposed to Huang the idea of making in China a documentary about Helen.

In September 1978 the team, consisting of Helen, Considine, a videographer and an Academy Award-winning soundman, was in Shanghai, just in time to celebrate Helen's 71st birthday.

"She came to life in China," said Considine, whom Helen took everywhere from the historic hotel in the old concession area in Shanghai to the couple's courtyard residence in Beijing. But the highlight was the Northwest, where Helen met An.

"Helen talked in a super fast pace and was always with her notebook," An said. "She was too old to climb the mountains she once climbed. But we went to see the little rammed-earth house she slept in in Yan'an, where dirt used to fall off every time a mouse scurried along the ceiling.

In Xi'an, Considine filmed Helen standing right on one side of a trestle bridge with a train roaring by, an apt metaphor for the flying of time. The bridge looked down to the north gate of the city through which the car carrying Helen, dressed as a man, traveled on that early morning in April 1937.