If these were not the bottom-drawer doldrums of another rerun summer, "Evening in Byzantium" would not be worth a passing glance, much less serious consideration. But this latest two-part, four-hour TV movie to huddle under Universal's "Operation Prime Time" umbrella may attract an audience just because it's there - and hasn't been there before.

There is, however, a deafening ring of familiarity to this flagrantly protracted production, which airs tonight and tomorrow night at 9 on Channel 20 and a number of other local stations throughout the country. Glen A. Larso and Michael Sloan's adaptation of an Irwin Shaw novel seems not so much ripped from today's headlines as it does ripped from yesterday's lousy movies.

The story is set at the Cannes Film Festival, where the filmmakers and the political terrorists allegedly play, and at one point a terrorist denounces a film that has been screened as "a piece of trash - shallow and uninspired." Not only is he the only terrorist to have his dialogue written by Judith Crist, but he is also given the honor of summing up the very movie in which he is appearing.

As has been all too firmly established in recent years, trash can be fun, and "Byzantium" may be foolish and preposterous enough to qualify as the kind of hapless diversion on which television thrives. The first half, though, is basically a tepid warm-up for the apocalyptic hijinks of the second, which is climaxed by a fight between Glenn Ford and Vince Edwards that, we have been told, will decide whether or not the world is to catapulted into a nuclear holocaust.

Ford is the good guy.As aging, long-dormant film producer Jesse Craig, he comes to Cannes with a script that is reverentially hailed by all who read it as "dynamite," "nitro," and "a powder keg." Indeed, "Evening Byzantium" itself has been tirelessly promoted as being "explosive." The word everyone seems to be avoiding is "bomb." It does come to mind, however.

There are no actual explosions until "special guest star Shirley Jones," as Ford's fickle but fanatically humanitarian ex-wife, steps into a booby-trapped bathrobe at the end of part one and thar she blows. The blast must have cost a lot of money because it is repeated three more times, in flashback, during part two. Soon all heck is breaking loose at the festival, where a pack of raggle-taggle terrorists led by mean old Bret Easton (Edwards) discovers that Craig's script contains a blueprint for nuclear blackmail identical to their own dastardly plan. We know Edwards is a heel because he opens the film by saying "America is finished."

The story is padded out with fleeting romances and turns of event more nonsensical than surprising. Exposition is prodded along through the character of a voluptuous journalist played by model Erin Gray, whose acting style combines the optical excesses of Karen Black and Barney Google.

This is quite a contrast to Ford, who sets a new standard of errant, sluggish somnambulism as he stumbles through his role. Eddie Albert exclaims to him," I don't think I've ever seen you this excited," you wonder who he's taking to. It isn't clear that Ford is even breathing. Perhaps he is trying to perfect a David Brinkley impersonation, but he has gone way beyond stoic. The role of producer Craig really calls for a grand ham along the lines of Kirk Douglas; Ford is lifeless.

If there are any people so bored as to be aficionados of this genre - the crummy TV potboiler - they will probably relish the inevitable dialogue howlers, most of which have been genersously lavished on newcomer Gray: "There's more than a story, here, Mr. Craig - there's a person's" and "This is real life, Mr. Craig - not the movies!" But then it's also something of a hoot to bear Ford warn that "the world's got to come to an end unless somebody does something about it" and say it will all the urgency of an accountant ordering lunch from a carry-out.

Jerry London, who also handled "Wheels" for Universal, directed "Byzantium." Never once did he allow imagination to compromise his efficiency. Among the whackadoodie devices employed is to inject a thoroughly gratuitous ersatz relevance to the story is a flashback to 1968 Bobby Kennedy campaign fund-raiser. Kenney is played, from the back only, by Nick Dyrenforth. Like all the others in "Byzantium," he is not part of a film, or a TV show, or an entertainment, but rather an item of commerce. Even at that level and even for free on TV, "Byzantium" is no bargain, although extremely patient viewers will be able to discern a yummy if inadvertent ripple running through its 10 tons of flavorless vanilla.