"After a while, you tap with the same person for months on end you can tap as rapidly as we are talking now — like the old telegraph operators in the old west,” McCain said during a question and answer session about his five years in captivity.

Brace, 81, called McCain — who was imprisoned starting in 1967 — “my insurance policy,” explaining, “I knew that if I went back to the jungle they could shoot me and dump me in a hole and say I was never there. But if they knew that I'd been talking to John McCainJohn McCainTrudeau, Trump speak for second night about US-Canada trade McCain: China has done ‘nothing’ on North Korea Trump administration weighing order to withdraw from NAFTA MORE, I knew John was going to be released."

McCain says a lot has changed since his days of being held captive in the country: "A ship named after my father and grand father, Destroyer USS McCain, visited the port of Da Nang not long ago, which shows that, if you live long enough, most anything can happen."

Despite the serious tone of the television special, there were a few smiles and even some lighthearted moments. At one point, McCain said of the footwear he wore while in Vietnam, “By the way, the sandals we wore were cut out of automobile tires. I highly recommend them, one pair lasted me five and a half years."

Photo: National Geographic Channels/Evelyn Hockstein