THE PLOT of "Evita" sounds like material for a stand-up comic: "Have yor heard they're doing a musical about Eva Peron? She dies of cancer in theend. All they need for a cast is the country of Argentina." But if you listen to composer Andrew Webber, who at the age of 33 has three enormously successful shows to his credit, it seems pwefectly reasonable. Webber and librettist Tim Rice were inspired by a desire to say something about political systems, the danger of demagoguery and extremism. Not exactly surprising material for drama, but unusual for a musical. "It is meant to be a cautionary tale," he said during a pre-opening visit to Washington from England. "If you leave the theater thinking, 'Hey, I went along with that lady's road show,' the show has made its point.

"At the time 1976 it was written against the background of people talking about setting up private armies and things like that . . . The whole point of 'Evita' is that what she did could happen here. The democratic way of life is something that can be very easily overthrown by an attractive extremist . . .

"You could say that there is no parallel between Argentina and England -- but Argentina was the most democratic country in S.A. at that time . . . You seem to be more stable politically here, but it's no good saying, 'It can't happen here.' I care passionately about the democratic liberal way of life. Any dogma is imperfect, but extremism is entirely imperfect."

The effect he wants the show to have was also inspired by an experience he had at a Judy Garland concert. "It was near the end of her life. She was 40 minutes late and she came on and sang 'Over the Rainbow' rather brokenly. The song had turned on her; the audience got ugly. She stayed 15 minutes -- and it was the most theatrical 15 minutes I've ever seen. Truly tragic. I wanted in 'Evita' to find a theme that would turn on her in the same way." That song is "Don't Cry for Me Argentina," although some might argue as to whether it has the intended effect.

Webber is physically slight, intellectually intense. He talks rapidly, and seems still something of the Oxford undergraduate. He published music when he was 9, and wrote his first show, with Rice, in 1967, at the age of 19. That was "Joseph and the Amazing Technicolor Dreamcoat," which took a circuitous route through the Olney and Ford's Theatre here before eventually getting a Broadway production this month.

Then came "Jesus Christ Superstar," a history-making opus in its own way. It started as a record, then became a concert version of the record, then a stage show and finally a movie. One of the songs, "I Don't Know How to Love Him," became a hit single. But the show's level of sophistication was nowhere near Webber's later works; its sensation was capsuled in the title, which offended a few but became a catchword for many. Each version was successively less successful: The record was very popular, the spin-off concert versions -- some of them bootleg productions that produced a lengthy lawsuit -- quite profitable, the stage show less so and the movie forgettable."The definitive production of 'Jesus Christ Superstar' is still to be given," he laughed. "We never got the right producer here . . .

"I was always worried about the success of 'Jesus Christ Superstar' in America," Webber said. "Since it was on record, I was afraid people would think I was just a pop music writer who got lucky. 'Evita' took a long time to break through here . . . I think we were paying dues for 'Superstar.' Now with 'Cats' (his current London hit), people are saying we must be serious about the theater . . ."

"Superstar" also did a lot to increase the use of microphones onstage, a phenomenon that has now gone beyond the bounds of effectiveness. Indeed, one of the reasons Webber was in Washington was to check the sound system for "Evita," to make sure the amplification did not create "a wall" of electronic sound. "People have stopped listening," he complained. "They expect the theater to sound like a recording. People mistake volume for energy . . ."

"Cats," his latest work, which Webber wrote without Rice, is based on T.S. Eliot's "Old Possum's Book of Practical Cats." All the characters are cats, played by people, and the orchestra includes seven synthesizers with a combined total of 40 memory banks.

"There are two kinds of musical theater," he said. "The most interesting work has been done by Stephen Sondheim. But I do enjoy a good old-fashioned musical like 'Annie' as well. The thing I find a little depressing about the Broadway season is they're so overloaded with nostalgia. It's all celebrities or stuff from the past. Rather incestuous. 'Evita' is at least new ground."

Recently Webber produced a single with Barbra Streisand, who saw "Cats" while she was in London filming "Yentl" and wanted to record one of its numbers. He had a copy of the song on a cassette in his coat pocket, and casually popped it into the "Evita" sound system after a rehearsal. He smiled slyly as the few people left in the theater gradually stopped talking and listened.

He will compose music for the opening of the massive Barbican performing arts center in London this spring, and will be back in the U.S. to supervise the Broadway production of "Cats" next fall. After "Evita" he left the aegis of the Robert Stigwood organization and formed his own production company, and he helped found an arts festival near his country home, where several of his works have been incubated. He has also written a full-length orchestra piece for his brother Julian, a cellist.

"I only want to do things that stretch me," he said with the modest self-confidence that is acceptable in one who was so successful so young. "Success or failure means less than other people might think. You obviously mind if people criticize you, but the sheer joy of doing it is the most important thing."