Call it the Sept. 11 effect. Since that dark day, even the most exaggerated, apocalyptic fantasies about terrorism and nuclear destruction have to be taken seriously.

In movie terms, what used to play as trashy, video game-style entertainment is now being held to the highest standards of political realism and intelligence.

The Sum of All Fears, the fourth film in a lucrative big-budget series based on Tom Clancy's best-selling novels, dramatizes some of these fantasies. Director Phil Alden Robinson (Field of Dreams) delivers the goods according to the old escapist rules — but his film falls short when measured against our post-9/11 expectations.

Filming was completed before the World Trade Center attacks, and Robinson's crew has succeeded in altering the movie's tone during the editing process to suit the new national mood. Yet Clancy's book was published in 1991. The political landscape has radically changed since then, and much of the plot now seems dated. But the filmmakers' “improvements” on the original story make matters worse as they conform to politically correct attitudes popular in pre-9/11 Hollywood.

Paramount Studios has been accused of crass exploitation for distributing the film at this time. But the pop-culture airing of our deepest fears about nuclear terrorism may be a good thing, providing a release of tensions that could have a salutary effect on our consciousness.

The movie begins during the 1973 Yom Kippur War. An Israeli plane carrying a small atom bomb is shot down over the Golan Heights. The bomb ejects and lands in the desert sands.

Some years later a pair of Palestinian Arabs discover the weapon and, unaware of its nuclear capabilities, sell it to a South African arms dealer (Colin Feore) for $400. He, however, knows what he's got and sells it for many more millions to Richard Dressler (Alan Bates), a wealthy Austrian neo-Nazi who dreams of resurrecting the Third Reich. “The virus doesn't need a strong host to spread,” he declares.

Flash forward to CIA headquarters in the late 1990s. Jack Ryan (Ben Affleck) is a novice analyst who specializes in Russian politics. His imaginative ideas catch the attention of the agency's veteran director, William Cabot (Morgan Freeman), who becomes his mentor. Clancy fans will note some significant changes to Ryan's character.

He has suddenly become younger and lost the spouse to whom he was married in his previous cinematic incarnations. Instead he's shown dating a beautiful physician, Dr. Cathy Muller (Bridget Moynahan), who may become his future wife.

This version of Ryan is an information-age, Generation-X yuppie with mildly rebellious ways — a very different figure than the mature family man of the earlier movies.

Although the new Ryan works hard and knows all the answers, his manner is cool and hip, and he dresses more informally than his coworkers.

The final countdown is triggered by Russian aggression in Chechnya. The filmmakers, along with all their American characters, view this purely as a human-rights violation. There's no mention of this aggrieved people's Islamic culture, or of its evolution into a breeding ground for terrorists.

Ryan is taken by Cabot into a national security meeting chaired by President Fowler (James Cromwell). The novice analyst alone believes that the Russian president, Nemerov (Ciaran Hinds), may not be as hawk-ish as his actions suggest.

Ryan is also part of a mission to the former Soviet Union in which it's discovered that certain key nuclear scientists have disappeared. In a convoluted series of plot twists, we learn that the neo-Nazi Dressler has made an alliance with rogue elements of the Russian military and plans to use the missing physicists and the black-market bomb to trigger a nuclear showdown between the United States and Russia. When the rubble clears, the new Third Reich will somehow take control.

The filmmakers keep us on the edge of our seats as Ryan, with some help from Cabot and an experienced operative (Liev Schreiber), tries to checkmate this scenario. In the process, the novice analyst must transform himself from a desk jockey into man of action.

In the movie's scariest sequences, Dressler's free-lance terrorists succeed in smuggling the missing nuclear weapon into the United States and detonating it during a football game at a crowded Baltimore stadium. The film handles the horrific mayhem that follows with appropriate restraint, resisting the impulse to milk it for exploitative effect.

Our leadership assumes that Russia is behind the attack, and the story's final third replicates the plot of the 1964 classic Fail-Safe, in which America and Russia inevitably escalate their moves to a terrifying showdown. In The Sum of All Fears, Ryan is presented as the only person who can save the world from nuclear obliteration.

Despite the high stakes, all this is a letdown. The U.S.-Russian confrontation seems left over from the Cold War, and the villains are irrelevant. In Clancy's novel, the terrorists were Arabs, a choice that now seems prophetic.

But Hollywood's fear of ethnic stereotyping has led them to make the bad guys neo-Nazis. This failure of nerve weakens our appreciation of the movie's considerable accomplishments as a suspense-thriller.

After the World Trade Center attacks, we believe that what the filmmakers have put up on the screen in The Sum of All Fears might someday happen to us.

But now we also demand a more sophisticated understanding of why.

John Prizer writes from Los Angeles.