Opera Theatre of Northern Virigina presents "The Jumping Frog of Calaveras County" by Lukas Foss, Sunday at 2 p.m. in the Thomas Jefferson Community Center, 125 South Old Glebe Road, Arlington. Admission.

If you mix together seven people, a story by Mark Twain and a singing group therapy session, what do you get?

The unlikely answer is children's opera -- specifically, a 45-minute version of "The Jumping Frog of Calaveras County," which German-American Lukas Foss based on Twain's celebrated story. The opera will be performed for both the general public and Arlington County schoolchildren by the Opera Theatre of Northern Virginia, starting this Sunday.

The company, which has eight years of experience in meeting the special needs of children, hopes through this performance to reach out to another special group; the hearing-impaired. Using equipment on loan from the Phonic Ear Co., the group has rigged stage microphones to deliver the opera to special receivers for this purpose.

Several school groups are already scheduled to bring their hearing-impaired children, but theater president Carol Pierson says the equipment may be made available by prior arrangement for the Sunday performance (call 243-5455 or 558-2165).

The opera is aimed primarily at elementary school-age children, and selected Arlington teachers have already received a guide to help them spark the interest of their students. For those whose attendance is prompted more by curiosity than a school bus, here is what to expect:

It's gold rush time in California, a fact that accounts for all seven singers' Mark Twain twang. Our hero (Smiley) has a masterful frog named Dan'l Webster. These two are in a saloon, where the frog displays his prowess before the bar's owner (Uncle Henry) and The Girl (Lulu).

In walks the bad guy (The Stranger), who, after some wheeling and dealing, bets Smiley that his frog can jump farther than Dan'l Webster. This occasions the opera's first group thereapy session (ensemble) wherein the characters pause to tell the audience how they really feel about the situation.

The Stranger, obviously up to no good, then cajoles Lulu out of an invitation to "dinner" (this is a family show). After she leaves, the villain takes a shotgun off the wall and stuffs the hapless Dan'l Webster with buckshot, to weigh him down.

This is followed by a private therapy session (aria), in which The Stranger tells us his grand and evil design: Each time I hit a town, I do the same; "I shake a feller down and jump the blame. "Find me a woman and give her the eye; "Then thank ye kindly, ma'am, and it's goodbye."

That brings us to Scene II, held in the village square, where two men are shooting craps and one is playing a guitar. Uncle Henry informs them of the upcoming frog-jumping contest, and suggests that they place their bets on a sure thing (Dan'l Webster).

The Stranger and The Girl enter and sing a duet all about the good dinner she made; this is overheard by the crap shooters, who tease the two unmercifully.

In comes Smiley and the weighted frog, and there ensues another group therapy session, in which we learn how everyone feels about the contest and each other.

The contest begins: The Stranger's frog jumps, and Dan'l Webster remains solidly implanted. That's as much as I'll tell you -- you'll have to go to the show to see how it turns out.

Even with -- the plot revealed, your children may still be confused by some of opera's more unbelievable conventions. Clark Dobson, a history professor at George Mason University who wrote the teacher's guide, suggests that these be discussed first.

"When we look at a painting," he says, "we accept the illusion of depth in spite of the fact that the canvas is flat. Opera presents a new sort of illusion that the viewer needs to become accustomed to before he can really begin to be captivated by the very specialized and heightened form of reality that opera has to offer."

In opera's reality, for instance, people sing to each other, an act that Dobson sees as a natural progression from talking. "Each syllable we speak has a pitch to it, though it is not as clearly defined as the technique of singing," he says. If you don't believe him, try saying "Good Morning" four different ways, and listen to the pitch levels of your voice.

Another element that removes opera from people -- especially little people -- is its complexity. As Arlington's fine arts specialist Larry Bohnart puts it, "Opera is the most extensive and collaborative type of performing art, incorporating acting, singing, music, set design and costumes." pThis makes it difficult to bring into the schools, so the schools, send the kids to the opera.

"Over the last eight years, all of our elementary schools have had a chance to see an opera," he says, "and now we're inviting students from the intermediate grades." He says he is particularly pleased with this year's opera selection, since it "correlates with the curriculum -- fourth and fifth graders study Mark Twain."

They also study music, but group therapy isn't a curriculum choice -- even if it's sung.