Who knows what urges surge through the heart of your homely lunchroom?

The Lafayette Cafeteria, that's who.

One floor down from its sibling, the Lafayette Restaurant, a couple of blocks from the White House and across the street and down a bit from the Decatur House, the cafeteria does its meek, mild-mannered business in under- three-dollar lunches Monday through Friday, serving tired tourists and those who work nearby. There must be a hundred such cafeterias in Washington.

But this one, on Friday evenings, steps into a phone booth, rips off its go-to-work sincere suit and turns into Brazilian Sundance, a Latin disco that can leap tall tangos at a single bound and draw crowds in a neighborhood that's otherwise deserted at night.

The scene is Latin, not Brazilian, but on Friday and -- especially -- Saturday nights, the name's no obstacle: Last Saturday night there was no Brazil and less sun, but lots of dance -- about 200 people hopping to the music of the Banda Ipanema, bumping up against one another, their spirits as high as the decibel level.

Less frenetic, says Mike Cinquegrani, who owns the place, is the Friday-night scene, which is when he'd bring his wife, say, to have a better chance at a table and calm conversation.

Only a few of the cafeteria's lunch crowd -- "about a dozen," Cinquegrani says -- have noticed or asked about the mirrored ball that hangs near the food line; they seemed satisfied by the explanation that the place is a Latin disco on weekends, and of course they never see the bandstand that goes up on Fridays and vanishes before Monday, or the dance floor.

There are a million stories in this city, a lot of them involving secret lives of people you may see every day; the tale of the Lafayette Cafeteria is just one of them.

Ole! BRAZILIAN SUNDANCE -- 1621 H Street NW. Admission $5, open 9 p.m. to 3 a.m. Fridays and Saturdays, and there are plans to open Sunday nights as well.