"Diner," a modestly budgeted film about french fries, girls and destiny set in the Baltimore of 1959, was a film without a future a month ago.

MGM had hoped that Barry Levinson's easygoing examination of life in the slow lane after high school would appeal to the youth market, but when ads emphasized its golden oldies score at test engagements in St. Louis, Baltimore and the drive-ins of Phoenix, the response was flat.

In Washington, where the film opened with no advertising support, it was greeted with a mixed review and theater audiences that numbered in the dozens. Even in Baltimore, where "Diner" was set, filmed and received a favorable notice, it failed to catch on. Word of mouth seemed to have gone to the dentist, and grosses at the Senator theater declined precipitously each week: $13,200, $8,900, $6,800, $3,800.

Very soon, "Diner" was not showing anywhere at all.

Today, this small film--the least expensive movie made by a studio last year--opens at the Jenifer and Springfield Mall theaters, trailing clouds of glory.

Jerry Esbin, president for distribution of United Artists and MGM, says, " 'Diner' is Lazarus."

Levinson, who wrote and directed it, reports "Diner" has now grossed about a million dollars in just seven theaters. As of today it opens in one theater each in 12 cities and will be in about 25 by June. "There's no question the studio will make its money back," Levinson says.

What brought "Diner" back from the dead seems to have been a combination of its own low-key charm, sympathy among powerful critics and a flexible marketing strategy made possible partly because of the film's low cost.

Curiously, bad movies are sometimes easy to sell and good movies sometimes difficult. "Conan the Barbarian," which also opens today, has received enormous pre-release publicity and, whatever the critics say, its opening business is bound to be brisk. "The Great Santini" and "The Stunt Man" are classic examples of good films that never found a wide audience. Nobody knows why, including their frustrated and defensive distributors: They just seemed to defy success. Nor does fortune frown only on artful, offbeat productions. "The Pursuit of D.B. Cooper," a fast-paced, commercial-minded thriller starring Robert Duvall and Treat Williams, disappeared without a trace last year, apparently overwhelmed by the dueling blockbusters of the 1981 Christmas season.

What pulls the plug on struggling movies is a standard rule of business: Don't throw bad money after good. In the light of "Diner's" disastrous opening weeks, MGM and UA prepared to write it off.

"Then we heard that the New York critics had seen it and were going to be favorable," said Esbin. "And we knew they were ready to blow us out of the box if we walked away from it at that point."

According to Levinson, "The New Yorker and Rolling Stone and the news magazines let it be known that they believed in the movie and were going to run their reviews whether it was playing anywhere or not. So MGM moved it into New York at that point."

In New York, "Diner" instantly turned out to have what Esbin calls "charisma." It did $50,000 in business its first week, and $70,000 the second--at one movie house. The New York Times ran a favorable review, followed by a lengthy Sunday piece about Levinson and his boyhood in Baltimore. Suddenly word of mouth had been activated and "Diner" had attained the status of a "sleeper."

As more critics and ticket buyers jumped on the bandwagon, the distributor did too. Levinson says he never was happy with the "youth-oriented" ads for his picture. "They were expecting 'Grease' and they didn't get it. But once a campaign gets going on one track you can't make them do a U-turn." When the campaign stopped dead it was apparently easier to turn it around.

"I do think our fault was in opening the picture without enough preparation," Esbin said. "Now we're screening the picture free for groups which can stir public opinion. We show it to colleges, high school groups, Rotary Clubs. And we've got a groundswell going. We really struck a nerve here. This may be something that will work for all pictures like this."

Such free showings permit audiences to decide for themselves the appeal of the film--and tell their friends. This is especially useful if the appeal is hard to put into a few words, the way "King Kong" can be. (Ape Menaces City!) A fan of "Diner" is more likely to hear himself saying, "Well, six guys hang around a diner at midnight, arguing about the Colts and girls and eating french fries, and you kind of get interested in what's going to happen to them . . ."

The relatively low cost of "Diner" worked in favor of its resurgence. "MGM knew it couldn't get seriously hurt," Levinson says. "It's not a gigantic gamble for them either way." But a studio holding a $20 million potential turkey thinks twice before spending an additional $10 million in distribution and advertising costs. The comeback of "Diner" has been virtually free. It still has no TV campaign and only minimal newspaper advertising.

If word of mouth failed at first, why does it propel the movie now?

"Well," Levinson speculated, "we sat in on it two days in Washington. There were 80 people in the theater and they laughed out loud and applauded. But 80 people telling their friends aren't enough. You have to have a larger base or nothing will happen."

New York gave "Diner" that base, and assured a run in 25 theaters around the country. But Esbin stops short of predicting that everyone in America will flock to the film.

"We don't know the real appeal yet. It has to get the kids in. They'll go see 'On Golden Pond,' where a kid says 'suck face,' so maybe they'll come see this. It has to survive all the other summer product coming in. If it does, we'll widen the run out even more."