Early in Eugene Jarecki’s documentary, “Reagan,” you hear the voice of Ronald Reagan saying, “Someday it might be worthwhile to find out how images are created  and even more worthwhile to learn how false images come into being.”

Indeed. The image that many, perhaps most, Americans have of the nation’s 40th president is largely manufactured. Reagan has become this larger-than-life figure who all but single-handedly won the cold war, planted the Republican Party’s tax-cut philosophy in the resistant soil of the liberal Democrats and is the touchstone for all things allegedly conservative, no matter how wacky or extreme.

Mr. Jarecki’s documentary does a first-rate job of respectfully separating the real from the mythical, the significant from the nonsense. The truth is that Ronald Reagan, at one time or another, was all over the political map. Early on, he was a liberal Democrat and admirer of Franklin Roosevelt. Reagan’s family received much-needed help from the New Deal during the Depression.

It is well known that Reagan was the head of the Screen Actors Guild. And though he was staunchly anti-Communist, he did not finger anyone when he appeared before the rabid House Un-American Activities Committee. But Mr. Jarecki learned that at the height of the Red Scare, Reagan had been secretly cooperating with the F.B.I. He was registered officially as Informant T-10.