Shots rang out in the dining room of the Georgetown Inn two weekends ago, killing the venerable U.S. senator from Montana in the middle of a speech announcing his intention to run again.

The stunned guests gasped, although most also kept right on eating their salads. The gunman had fired from the hallway and escaped unseen. And by the sheerest coincidence, all the major suspects were out of the room.

After all, it couldn't have been any of the more than 30 guests who had paid $40 a plate for the "Mystery on the Menu" dinner, a new feature at the inn this summer.

The idea was concocted by writer/director Barbara Fox, the Georgetowner who has built a business on daylong crime-solving excursions to New York aboard the Zephyr Express. Other Fox audience-participation mysteries have been presented in single-night performances around town and recently on train trips to Atlantic City.

Capitalizing on the continued popularity of crime-solving as entertainment, the format gives whodunit lovers and amateur sleuths the chance to solve their own cases. Guests can comfortably participate or spectate, depending on their sense of humor and theater.

Fox's Georgetown Inn scenario paints archetypal Washington characters with a broad brush. Patrons have little trouble identifying the cast among them -- or comparing theories about the killer.

Was it the senator's fair and footloose young bride? His disinherited singer son? The campaign manager, who has discovered some missing funds? The Washington hostess who had her sights set on the senator? Or the reporter who knew too much about all of them?

Fox plays the jilted hostess. A commercial writer for years, she turned to fiction, then hit upon the idea of audience-participation plays. She tested her concept with 150 guests at a friend's resort in Coolfont, W.Va., and decided she had a successful formula.

She maintains that no two nights are ever the same. Though guests don't decide the ending as they do in the Kennedy Center's "Sheer Madness," they "sure affect the way we get there," she said.

To get there, Fox uses seven innovative local actors. They not only know their lines, but also have to constantly ad-lib new ones as they dine with the guests, drop clues and answer questions.

Some guests really get into it too. When actor Steve Kramer (the reporter) went from table to table gathering dirt on the suspects, he sometimes got more than he bargained for.

Washington lawyer Francine Augustyn, for instance, launched into an engaging rundown of her lobbying efforts with the slain senator. By the main course of boneless chicken with potatoes and champagne, even initially quiet diners were testing their hunches.

"Isn't it true you knew the bride before she met the senator?" a prim elderly woman asked as disarmingly as Miss Marple.

"You thought he was going to marry you, didn't you?" demanded a bespectacled Perry Mason type.

"Just how much is missing from the campaign fund?" inquired his redheaded dinner date.

Fox said the scenarios require extensive rehearsals so the characters don't contradict one another. "I don't want anyone surprised by an affair he or she didn't know they were having," she said, smiling.

But sometimes the unforeseen occurs anyway. Like when an actor rushed into the hall one night in early June crying, "The senator's been shot!" only to return seconds later pursued by two D.C. police officers with guns drawn.

"We had to say, 'Hey, we're only playacting!' " Fox recalled, "which kind of shattered the illusion."

Two weeks ago Fox's daughter, Marci McDonald, playing a detective school dropout, brought in two Georgetown Inn security guards to give her denouement extra weight.

Her case depended on a coded message dropped, of course, by the killer. The code was easily cracked by a Sidwell Friends sophomore at table No. 2.

For actor Sam Polson, the plum role is that of victim. "You get a lot of attention and get killed early," Polson said. "I usually go home right after I get shot."

But the Georgetown Inn stint is an interlude, Fox said, and the troupe resumes its train trips to Atlantic City on Aug. 5. The trips, like the Georgetown dinners, are murder. But murder, she wrote, as fun.

The show continues at 7 p.m. Saturdays through July at the Georgetown Inn, 1310 Wisconsin Ave. NW.