On June 5, 1989, Jonathan Chan arrived at Beijing airport with two swollen bumps visible on the back of his head. In a line of tired and weary student demonstrators, eager to slip out of the capital if not the country, the 24-year-old slid his hand into his pocket, fingering the single roll of film he had rescued moments before his camera had been smashed by Chinese soldiers. A journalist standing next to him took notice, quickly explaining how he might break open the roller, overexpose the images, and protect himself if he were stopped by airport immigration. Then Chan walked to the front of the line and waited for the inevitable question.

“Were you on the square?” the immigration official asked.

“I was,” Chan said, expecting to be detained.

Frowning, the official leaned in.

“Then go and tell the world,” the official said softly, before waving him through.

For the past 25 years, that’s exactly what Chan, Lam and Lee have tried to do. As leading student activists from Hong Kong, they played a vital role in the protests at Beijing’s Tiananmen Square, not only as fundraisers and couriers of supplies for their fellow protesters, but also as emissaries of information, able to evade Communist Party censors by returning to Hong Kong — then still administered by the British and enjoying a vigorously free press and communications regime — to share their stories...