The script is corn-syrupy enough to be right out of The Waltons. The guy is young and handsome, with freckles and red hair, a star in a TV series about an all-American family. The girl is a stunner, not in showbiz but wants to be, and she began watching his program all the time because she had a thing for him.

They meet cute: He's In bed recovering from mono, and a friend brings her by. There's a spark, and immediately he starts to get better. Within two months she's moved in with him-bringing along her 300 stuffed penguins. It's got to be serious. They decide to marry because, he explains, "We don't want any part of the Lee Marvin fiasco." The actor who plays the guy's father on TV - he's a min­ister on the side - performs the wedding ceremony in front of the whole cast.

In fact, the story is right out of The Waltons. The groom is Jon Walmsley, 23, who's been Jason Walton, John­-Boy's kid brother, since the series' premiere in 1972. He and his bride, Lisa Harrison, 23, daughter of actress­ turned-scenarist Noreen Stone, had discussed marriage so often during their three years of living together that she didn't quite believe his proposal. Just to make sure he was serious, Jon relates, "She made me get down on my knees.

Their wedding was a family-rated special: Ralph Waite, Jon's TV dad and an ordained United Church of Christ minister, led them in their vows. "I didn't know whether I was the father, the minister or a friend," says Waite. In addition to the families, the entire Wal­ton cast showed up at the outdoor ceremony in Malibu. Michael Learned (who announced her retirement as Mama Walton last season only to change her mind) noted maternally that the union was "made in heaven - they're so right for each other." Mary Elizabeth McDonough, who plays Erin Walton, saw the ring on Jon's finger and burst into tears. "After all," she said, "he's my older brother."

At the reception at Learned's house other luminaries dropped in, including John Ritter, who played a minister on The Waltons some years before Three's Company and joked that he should have officiated at this gig. Doobie Brother Pat Simmons, with whom Lisa and Jon hope to collaborate on a record, joined several other guests in performance. The young couple did a number which Jon introduced: "We'd like to do a song Lisa and I wrote together. It's called Cuddle Up, which is some­thing we can do a lot more of now that we're married. It'll be legal and everything."

In between Jon's work on The Wal­tons, he and Lisa have assembled a group to play clubs and spots like Dis­ney World. "It's not to make a fast buck because of The Waltons," says Jon, who plays 15 instruments. "Music is my first love and always has been. If The Waltons were to end, I'd work on mu­sic full-time."

Jon and Lisa's scenario may harmo­nize elsewhere, too. Next season one possible twist calls for Jason Walton to join the Army and meet a girl he gets "very involved with," says producer Rod Peterson. The choice of Jon Walmsley's on-camera love is positive­ly Waltonesque: It will mark the acting debut of his wife, Lisa.