Not only did Flood refuse to go, but he went to his personal lawyer and then to Marvin Miller, founder and executive director of the Players Association, and told them he wanted to sue Major League Baseball. The decision sent shock waves not only through baseball but ultimately through all professional sports. Those waves reverberate even to this day. And though he was aware that it would cost him dearly, he never wavered.

In 1969, players were still bound to a team for life by the so-called reserve clause. Simply put, a player was a team's property. Unless the team chose to trade him or release him, his first big-league team would be his only big-league team for his entire career. A player's only recourse was retirement.

Oddly enough, the language of the reserve clause was ambiguous. It merely said that if you played for a team, you must play for that team the next season as well. Two players before Flood had challenged the reserve clause only to run up against baseball's exemption from antitrust laws, first established in 1921 in a decision by Supreme Court justice Oliver Wendell Holmes.

The reserve clause, the antitrust exemption, and the legal decisions that had accumulated around it all had an aura of irrationality about them. It was as if, as Miller once put it, "the courts were saying 'Yes, you're an American and have the right to seek employment anywhere you like, but this right does not apply to baseball players."

When Flood came to Miller, his mind was already made up. "I told him," recalls Miller, "that given the courts' history of bias towards the owners and their monopoly, he didn't have a chance in hell of winning. More important than that, I told him even if he won, he'd never get anything out of it—he'd never get a job in baseball again."

Flood asked Miller if it would benefit other players. "I told him yes, and those to come.

He said, 'That's good enough for me.'" When Miller realized that Flood understood the odds against him and was still determined to go ahead with the case, he told him, "You're a union-leader's dream."

All these moments are captured in the remarkable new HBO documentary, The Curious Case of Curt Flood, which premieres Wednesday at 9 p.m. Eastern. The intriguing title is explained in large part by the fact that Flood's real motivations for his decision have never been fully explored in any documentary. Ken Burns' award-winning PBS series Baseball, paid tribute to the historic importance of Flood's suit but gave scant attention to the backstory.

Born in Houston in 1938, Flood was raised in the relatively tolerant Oakland, California. His mother, who had fled the intense racial bigotry of the pre-World War II South, never let him forget what things had been like where she grew up, and in 1962, having little idea of what he was about to encounter, the 24-year-old Flood went to Mississippi to join his idols Dr. Martin Luther King and Jackie Robinson to support the non-violent protests organized by the NAACP.