Boy meets Girl -- meets her plane, actually, at National Airport yesterday -- and drives Girl to town. Boy is hungry, so they stop at McDonald's. (Boy has a cheeseburger, Girl a Filet-o-Fish and chocolate sundae.) Boy brings Girl to a building under construction at 1300 L St. Girl looks up and sees a 3-foot-by-12-foot sign on the third floor. Sign is shocking pink and it certainly is shocking Girl. Sign says: "Lisa Marry Me!"

Girl cries. Boy grins and gives Lisa diamond ring. He is Pete. Pete says, "I love you." Lisa says, "I love you." They kiss. Workmen cheer. They do, or at least, they will.

Pete Mellits and Lisa Hughes met at a Halloween party at Old Dominion University in her home town of Norfolk. She, a dental-hygienist-to-be, was dressed as . . . "nah, skip that." He, a civil-engineer-to-be, had a leg injury from football. "He couldn't dance," recalled Hughes yesterday, "but he sure could slow-dance."

Mellits, 21, of Bowie, had the sign made up at Banner Sign Co. in Alexandria a month ago.

"Well, you're good at keeping secrets," his 21-year-old fiance'e replied. Lisa, wearing a pink skirt, matched her sign.

*Mellits' mother Christa was on hand but out of sight for Lisa's encounter with the sign. Christa Mellits, a Berlin native, doesn't remember how her GI husband Allyn, now an industrial fire extinguisher supplier, proposed to her, except that it was "in Germany a long time ago." She said her son is "very romantic" and her daughter-in-law-to-be is "really nice."

Also on hand was site manager Lloyd A. Kinch. Mellits has a summer job with Kinch's company, Sigal Construction Corp. When Mellits asked for this favor, Kinch said, "Great!" "Everybody on the job helped me put it up," Kinch said. "One of them asked me if he could put a second banner up. I told him I'd have to review it first. Some of these guys are a little salty."

One of the workers, Lundy Sanders Taylor III, 25, understood the hormonal urge that drives men to win women's hearts by putting their self-respect on the line. "I've done crazy things like that before," said Taylor. He put his wife's engagement ring in a Cracker Jack box. "I opened the prize packet and put the ring in there -- steamed the box open and sealed it up and said, 'You can have the prize.' " They had their first baby three days ago.

Female passers-by had mixed reactions. What would Felicia Wardrick, 29, say if her boyfriend did that for her? " 'Yes.' I think it's very romantic." What would Lisa Spriggs, 18, say? "I'd laugh. I'd say, 'Where's my ring?' "

Otto Adams, 46, thought he knew all about love after 18 years as a construction worker. "I've seen 'em go around with planes and all of that." But when he watched Mellits and Hughes from the third floor, "it was real emotional, man. It brought tears to my eyes."