In the towering mountains of northern India, a chilling chapter was written in the history of international espionage.

After the Chinese detonated their first nuclear test in 1964, America and India, which had just fought a border war with its northern neighbor, were both justifiably concerned. The CIA knew it needed more information on China's growing nuclear capability but had few ways of peeking behind the Bamboo Curtain. Because of the extreme remoteness of Chinese testing grounds, conventional surveillance in this pre-satellite era was next to impossible.

The solution to this...