"Public Enemies" is a very good gangster movie and a decently accurate retelling of the final year in the life of John Dillinger, the Depression-era bank robber who was the first "Public Enemy No. 1."

But it isn't is a full or accurate adaptation of Bryan Burrough's 2004 bestseller that gives the movie its name. The first comprehensive account of the Midwestern bank robbers and the simultaneous growth of the Federal Bureau of Investigation into a national police force, "Public Enemies" runs 624 pages - by far too long for any film.

So director Michael Mann and his co-writers cut the book down to the Dillinger story. In doing so, they largely cast aside the other gangs who hit banks across the Midwest in the early 1930s and lost the perspective needed to understand how the FBI bumbled its way to power.

There is zero mention of Bonnie and Clyde in the movie. "Pretty Boy" Floyd makes the film for all of about 30 seconds, gunned down by G-Man Melvin Purvis in one of the picture's opening sequences. Alvin Karpis and the Barker Gang turn up a couple times, plotting robberies with Dillinger but never executing anything.

The truncation, while necessary, makes it seem like Dillinger (Johnny Depp) was the only target of and the reason for the FBI's anti-bank-robbery crusade. In fact, that effort began after the "Kansas City massacre" of 1933, in which four law enforcement officers were killed by a gang led by Vernon Miller in an effort to free a federal prisoner. The early target of the investigators: Charles Arthur "Pretty Boy" Floyd - who probably didn't take part in the massacre.

That backstory would help explain why Purvis (played by Christian Bale) takes out Floyd early in the movie. But in opening the movie with Floyd's death, Mann has significantly altered the history of the bank-robber era, Purvis' story and that of the FBI. Floyd was shot, by exactly whom remains unclear, on Oct. 22, 1934. Dillinger was killed outside the Biograph Theatre in Chicago on July 22, 1934.

That's right: The shooting that opens the movie didn't really occur until three months after the picture ends. Purvis was at both shootings, getting credit for eliminating Dillinger, then falling out of favor with imperious FBI director J. Edgar Hoover (Billy Crudup). Hoover banished Purvis from Chicago, and it was by chance he was in Ohio when Floyd was spotted, tracked down and killed.

What Mann does get right is the incompetence of the college boys Hoover picked to staff the early bureau, and Dillinger's violent trail from his September 1933 Indiana prison escape to the shooting outside the Biograph. The major events of those 10 months - his escape from an Indiana jail using a wooden gun and the FBI's failed attempt to capture him at Wisconsin's Little Bohemia - are particularly well depicted.

"Public Enemies" being a feature film, not a documentary, Mann plays up the romance between Dillinger and Billie Frechette (played by Marion Cotillard) far beyond anything in Burrough's book. The love story on screen could have taken place. But it's a production of Hollywood imagination, needed to give the movie some emotional heart.

Neither the lack of backstory nor the changing of history in Floyd's death alters the fact that "Public Enemies" is a very good movie. But with a few opening lines and a willingness to follow the facts, Mann could have made a great film that would have been as accurate and informative as it is involving.

Reach L. Kent Wolgamott at 473-7244 or kwolgamott@journalstar.com.