“The Untold Story of Emmett Louis Till” is an important movie, but it’s not a very good one. In 1955, Emmett Till was a 14-year-old African-American from Chicago visiting his cousins in Money, Miss. He made the mistake of whistling at a white woman. A few days later he was kidnapped, tortured, and murdered; a sham trial followed, and the defendants were acquitted.

This documentary, 10 years in the making, has uncovered new eyewitnesses, new evidence, and new defendants. The director, Keith Beauchamp, even convinced the Department of Justice to reopen Till’s case. This is an admirable work of retribution for an un-avenged crime. But as a documentary, it’s pretty shaky.

Too much of the story is glossed over, and too many events go unexplained; the movie only lasts 70 minutes. Mr. Beauchamp, who at 34 is already penning his memoirs and has optioned his life story to Hollywood, excuses the movie’s deficiencies with the explanation that, due to pending prosecution, the whole story cannot be told. That accounts for the missing story, but not for the missing storytelling craft.

What redeems the documentary are Mamie Till-Mobley and Mose Wright. Mrs. Till-Mobley, Till’s mother, demonstrates a steely resolve; there may be no three minutes of film more shattering than listening to her catalog the horrendous injuries inflicted on her son. Wright, Till’s great-uncle, is seen only in grainy archival footage. But when the time came to go to trial, Wright put his life on the line and testified.

Getting to know these two individuals makes you wish this story had been left in the hands of a filmmaker, not a crusader.

Until August 30 at Film Forum (209 W. Houston Street, 212-727-8110).