In many ways, it makes "The Day After" look like a day at the beach.

"Threads," the BBC version of the end of the world, airs on Channel 5 Monday night, at the same time it's broadcast on stations in Chicago and Boston. It will be followed by a panel discussion hosted by Harvard law professor Arthur Miller.

The film, following the plights of two British families overwhelmed amid nuclear warfare, sounds much like "The Day After," the ABC version of the end of the world that riveted huge numbers of Americans to their chairs in the fall of 1983. Comparisons of the two were inevitable when "Threads" debuted in this country some months ago on Ted Turner's WTBS Superstation. The term most often applied to "Threads" is gritty. Not that "The Day After" made nuclear holocaust look slick, but "Threads" makes nuclear war look like something out of a grainy black and white photograph even though it was filmed in color.

It's not by accident that there's a difference between the two productions.

"We tend to make points not by having experts describe what's going on," said "Threads" producer Mick Jackson, referring to the way "Day After" actors like Jason Robards often described the post-blast phenomenon the viewers were watching. Jackson said his "Threads" project almost came unraveled when they heard about ABC's project. But his feelings changed after he saw it.

"We thought they were not doing justice to some of the issues. We felt they were skirting them," he said. "It comes down to the difference between American and British productions. I thought "The Day After" was brave, especially in prime time. But the verbalizing which goes on -- even after the disaster -- didn't seem natural."

Jackson brings to "Threads" the British dramatist's notion of social realism. "Ours is a more gritty film," he said. "Our characters don't tell the plot or how they felt about what's happening. We took care not to say anything that was not supported by research."

This translates into a film in which very ordinary British people are cinematically marched through the rigors of pre-nuke anxiety and concern and post-nuke shock and dispair.

And it was that human reaction to catastrophe -- and nonreaction -- that fascinated Jackson as he researched the question of what happens if someone drops the bomb. How do people react to a crisis that has no end?

Jackson has looked at the psychological studies. "When there's an air disaster," he said, "people are there with blankets and hot coffee and begin to integrate them back into society." But in a large-scale disaster, there's no one to offer tea and sympathy, no welcome wagon to the nuclear age. "There's no one there to assure them that things will get better. Apathy and a feeling of inability or lack of will to cope sets in."

To catch the feeling of ordinary people being put to an extraordinary test, Jackson enlisted novelist and playwright Barry Hines. His book, A Kestrel for a Knave, was turned into the film "Kes," as were "The Gamekeeper" and "Looks and Smiles." "He was initially reluctant to collaborate," said Jackson, who persuaded Hines that his ear for simple dialogue and his lack of familiarity with the intricacies of the nuclear question would work out just fine.

Jackson, who has been a senior producer at the BBC since 1967, brings the background of a technician and dramatist to his work. He started out studying electronics and switched to drama in college. His production credits include "The Ascent of Man, "The Age of Uncertainty" and "Connections." One of his next projects deals with the matter of the relative peace of the world over the past 40 years.

If he has a mission behind his ork, he said, it is one of "truth-telling . . . We're telling people the question is urgent. Now we have to say, 'What can we do?'