We wanted to believe in him from the beginning. It was July 3, 1999, and the man had suffered testicular cancer and nearly died at 25, but managed to come back and qualify for one of the most physically demanding races on Earth. Now he sat on his bike in the start house for the prologue of that summer’s Tour de France, a five-mile dash around a lame French amusement park called Futuroscope. As he rolled down the start ramp and gained speed, whipping his bike back and forth and sprinting out of the corners, it was clear he was on fire. No American had ever come close to winning the Tour’s prestigious first stage, and yet he won easily, thrillingly. It was the start of something huge.

Then, as required of every Tour de France stage winner, Armstrong gave a urine sample that afternoon, and it tested positive for a banned substance—corticosteroids, a catabolic agent used by athletes to aid recovery and increase endurance. That might have been it for Armstrong’s Tour and racing season, and it might have cost him his future—no seven Tour wins, no jillions of yellow bracelets. But race officials accepted his word (and doctor’s prescription as evidence) that the steroids in his urine were from a skin cream for saddle sores. And as there was no effective test for erythropoietin, or EPO, a prohibited blood-booster that pro cyclists took like candy in the 1990s, lab technicians failed to find the EPO in Armstrong’s system that day for another six years, and under circumstances that made it possible for him to discredit the finding.

Armstrong, of course, went on to win the 1999 Tour—and six more. Even more incredibly, though he’s been tested at least 200 times for performance-enhancing drugs, he had never been caught and sanctioned for doping even as, one by one, nearly all of his main rivals and several former teammates tested positive, served suspensions, or retired under a cloud of suspicion. Throughout an era of highly sophisticated doping, when the ...