It has played on news shows, and been prominently featured in the movie's trailer, but when you see it for yourself it's a sickening jolt of horror. The bomb explodes in the Baltimore stadium where the Super Bowl is under way, shooting outward in a rippling horizontal jet of radioactive waves. The Sum of All Fears, to its credit, doesn't fetishize the devastation -- doesn't turn it into a kicky and prolonged special-effects wow, like the ocean blast in ''Deep Impact.'' When CIA analyst Jack Ryan (Ben Affleck), who has been working to cool the tensions between the United States and Russia, looks up from the ground after being knocked out of his helicopter, he sees a mushroom cloud, but compared with all of that mythical postwar documentary footage of destructive desert blooms, this one is black and fiery and rather modest in scale.
You can feel the audience suck in its collective breath and hold it there. For the first time, perhaps, since the era of ''Fail-Safe'' and ''Dr. Strangelove,'' a major Hollywood thriller has shown us the unimaginable at a time when the reality of world events has shown it to be anything but unimaginable. ''The Sum of All Fears'' is a world-detonation thriller, at once urgent and lazy, that benefits from its connection to current events and also, by the end, suffers from it. The first half has been put together with a good deal of paranoid urgency. Once it gooses our deepest dread, though, the movie has nowhere to go. It gets mired in hand-me-down Cold War stodginess, exploiting the very tensions it has amply demonstrated to be all but obsolete. The threat of megalo-terrorism gets us right where we live, but the prospect of East and West locked in a countdown to World War III feels like a rerun.
Tom Clancy's novel, the fifth in his series of Jack Ryan thrillers, was published in 1991, and it was well before Sept. 11 that the film version was conceived, cast, shot, and slated for release. It would be easy, and in a sense accurate, to describe ''The Sum of All Fears'' as a competent, unexceptional nuke-on-the-loose espionage thriller that looks more eerily prescient, and about a dozen times scarier, than it might have had it come out a year before. Yet if the timing amounts to a kind of grisly -- and, it must be said, highly marketable -- coincidence, let's give Clancy and his adapters, director Phil Alden Robinson (''Field of Dreams'') and screenwriters Paul Attanasio (''Quiz Show'') and Daniel Pyne (''Doc Hollywood''), their due: The scenario they unveil touches a nerve of creepy-crawly alarm because, in the act of dreaming it up, they touched what had been festering in the violent underground of the world stage.
In ''The Sum of All Fears,'' the U.S. and Russia, having arrived at a state of uneasy alliance, find themselves on a myopic collision course, and that's because neither country's leadership can see the forest for the nukes. The real villain, apart from the abstract demon of mistrust, isn't a nation-state at all but an Austrian neo-Nazi, played by Alan Bates in a complicated beard as one of those cultivated paragons of movie evil who listens to opera as he plots to destroy civilization. Describing the free-floating nature of 21st-century fascism, Bates, in his best springtime-for-Hitler accent, says, ''The virus does not need a strong host in order to spread.'' In ''The Sum of All Fears,'' this apocalyptic bandit, who buys a rogue nuke dug out of the Golan Heights (it got lost there during the 1973 Arab-Israeli war), may look like the purest Hollywood, but he's a slippery-enough nihilist to function as a stand-in, a de facto metaphor, for our own current enemies.
It turns out that Ben Affleck is more than a stand-in for Harrison Ford. The smartest thing about the movie is the way that it reconfigures the character of Jack Ryan from Ford's gruff Bondian family man into a stubborn young information-age geek. The role has been tailored to Affleck's quick-draw glibness, and he's funny and sly as Ryan gets tapped, from the think-tank bowels of the CIA, to accompany Agency director William Cabot (Morgan Freeman) as he meets the new, proud, Putin-like Russian president (Ciarán Hinds, a dead ringer for former Velvet Underground rocker John Cale). Freeman always manages to do something new -- cast here as a man of immeasurable power, he speaks with dramatic low-volume calm -- and he and Affleck demonstrate a seasoned team's nimbly cutting spontaneity.
The movie would have been smart to keep Ryan in the high-pressure control room. Alas, he's sent out on a mission, and the desk jockey suddenly has to draw on his background as a Marine and get into the action. It's a big mistake: Affleck, despite his rangy frame, doesn't come across as a very physical guy -- that's where Ford has him beat -- and the second half of ''The Sum of All Fears'' lacks either the escalating grip of ''Crimson Tide'' or the chesslike political intrigue of ''Thirteen Days.'' Instead, it falls into a murky chasm somewhere between the mind of Tom Clancy and the real-world terrors that now trump even his showiest doomsdays.