It will require the suspension of all disbelief to swallow the latest filmization of one of Tom Clancy’s novels, in which Ben Affleck–to the discomfiture of some–has replaced Harrison Ford (“Patriot Games,” “Clear and Present Danger”) as the heroic Jack Ryan (a role originally played by Alec Baldwin in “The Hunt for Red October” way back in 1990). “The Sum of All Fears” was published as long ago as 1991, at a time when the Soviet Union was still in existence and the threat of a nuclear exchange between two superpowers was still quite real. Adapting it for the screen a decade later has required heavy revision, and frankly the effort by Paul Attanasio and Daniel Pyne proves unavailing. The result might seem timely in view of the fact that the story involves a horrendous assault on the United States, involving a nuclear explosion no less. But the plot isn’t about terrorism in any realistic sense. Essentially it’s still a Cold War tale, and as such it’s positively antediluvian. If one looks for antecedents, the most obvious is “Fail-Safe.” (Shards of other pictures–“Black Sunday” and “The Boys from Brazil” among them–pop up, too.) But Sidney Lumet’s picture was hoary and outdated even at the time of its original release; by sending up the same basic scenario “Dr. Strangelove” rendered it immediately obsolete. One can understand an effort, like that undertaken by George Clooney in 2000, to remake “Fail-Safe” for television as a sort of nostalgic period piece. But dressing up its plot in modern trappings and trying to pass it off as a tingling contemporary thriller constitutes a gross miscalculation. In the final analysis, “Sum” just doesn’t add up.

In the first place, one has to set aside all memory of the Harrison Ford-Jack Ryan flicks and accept that the character is now a 28-year old who’s just joined the CIA as a humble analyst in 2002. One also has to accept that a new president has come to power in Russia–a little-known but potentially bellicose fellow named Nemerov (Ciaran Hinds), on whom the boyish Ryan just happens to have written a thesis. This fact leads CIA Director Bill Cabot (Morgan Freeman) to call the newcomer in to help brief President Fowler (James Cromwell) on the implications for the U.S. Tensions between the two countries soon escalate, eventually leading to violent incidents which could culminate in a nuclear exchange–especially after a football game being attended by the president is targeted for an apocalyptic attack. What neither side realizes is that there are other players involved–a shady illegal arms dealer (Colm Feore) and a slimy Austrian neo-Nazi named Dressler (Alan Bates) whose goals would be advanced by a confrontation between the nuclear powers. It’s up to our hero Jack, new kid on the block though he may be, to persuade the president and his senior advisors to choose the path of restraint while hoping that Nemerov can do likewise against the demands of his more belligerent comrades. He gets an assist from a James Bondish U.S. agent (Liev Schreiber); on the other hand, he has to turn aside from his duties every once in a while to fret over the fate of his fiancé, the lovely Dr. Cathy Miller (Bridget Moynahan). After all the huff and fury are over, there’s a tacked-on epilogue that’s blatantly derivative of the close of “The Godfather.”

Despite the fact that this plot includes what amounts to a terrorist assault on America, it nonetheless comes across as a relic from a bygone era, and its convolutions–particularly those involving the malevolent Dressler and his stereotypical cohorts–feel silly rather than plausible. Still, the cast treat it as seriously as they can. Affleck is okay, but his performance represents a distinct letdown after his fine turn in the recent “Changing Lanes,” and he hasn’t the chops to pull off the denouement, which involves Ryan’s impassioned pleading for calm over a radio (in fairness, no actor could have sold the scene). Freeman contributes his usual note of world-wise gravity, and Cromwell gives the president more shading than the script provides. (It’s amusing to note that he and Donald Moffat, who was the corrupt chief executive in “Clear and Present Danger,” rather resemble one another.) One has to hand it to Hinds for learning some Russian to play Nemerov, but the role barely taxes his talents, involving little more than looking sternly pained and wearing his hair slicked down in a fashion that recalls the Brylcream generation (even Russian politicians nowadays seem aware of the value of more natural coiffure). Schreiber strains to act the hard-nosed agent, though his light comic tendencies get in the way, but reliable veterans Philip Baker Hall, Ron Rifkin and Bruce McGill do all that’s expected of them as the president’s chief advisors. The saddest assignment surely falls to Bates; he’s a fine actor who, in his later years, should be treated as a thespian treasure and given challenging roles, but in recent days he’s been reduced to playing a standby of bad horror movies, he eccentric expositor of the supernatural (in “The Mothman Prophecies”) and now the same sort of conspiratorial leftover from the Hitler years that Gregory Peck attempted in “The Boys from Brazil.” Bates is a far better actor than Peck, but fails just as signally to invest the ludicrous part with any real sense of menace. (In fact, the only person who pulled off a similar role was James Mason, also in “Brazil,” who got by simply by winking at audience to signal that, like them, he recognized the absurdity of it all.)

It remains only to point out that technically “The Sum of All Fears” has been very proficiently made. Though director Phil Alden Robinson lingers a bit too much over the more intimate scenes, he handles the action stuff well enough, and editor Neil Travis manages to keep the convolutions of the plot fairly clear. The special effects, as well as the cinematography by John Lindley, are also good. But the background score by Jerry Goldsmith, one of the great film music composers and one who can usually be counted on to contribute something of interest even to a mediocre project, is disappointingly nondescript. In this case–as with the recent “The Last Castle”–even he seems merely to be going through the motions.

If you’re in the mood for some really old-fashioned international skullduggery and undercover sleuthing, therefore, this newest adaptation of a Tom Clancy potboiler might hold your interest. But you could just rent a video of “Fail-Safe” and take your nostalgia straight. Or better yet, watch “Dr. Strangelove” again and appreciate the work of an artist who recognized even forty years ago how nonsensical this sort of thing would appear when treated without even a hint of irony.