Open Monday through Saturday, 11 a.m. to 3 p.m. and 5 to 11 p.m.; Sunday, 3 to 10 p.m. No reservations for groups of less than 5 people. AE, MC, V. Prices: Main courses at dinner, with soup or salad, $5.25 to $12.50, average $7 to $9.Children's dinners, including dessert, $2.

Just as nobody purposely rears an ill-tempered child, nobody plans a bad restaurant. The budding restaurateur dreams of a beautiful dining room where charming waiters efficiently serve steaming plates of food that tantalizes the eyes and tongues of appreciative and good-natured clients, hordes of them. But the path to reality is rocky, and many a restaurateur, like many a parent, never even recognizes that he has indeed raised an unruly offspring.

Plata Grande still is the handsomest restaurant in Prince George's County and can still compete visually with most of the metropolitan area's best-dressed dining rooms. But after a promising early start, a revisit after two months showed the food slipping seriously.

There is still a lot that pleases at Plata Grande. Like a cactus flower in the desert, this rambling hacienda is a vivid touch in a field of strip-shopping and motels. Inside, the entry is a fiesta of red tile and fountains, merely the first of several delightful rooms festooned with silk banners and hanging greenery. The mode is, of course, Mexican, in kindergarten-bright colors. Between the enormously high ceilings and the earthy tile floors are bouquets of brilliant paper flowers, blown glass lamps, curlicued ironwork, a showcase of Mexico's extraordinary folk art. Even the waitresses complement this gallery, wearing loose, multicolored dresses and yarn braids in their hair. This restaurant looks delicious.

Plata Grande was born into an ambitious family. The sumptuousness of the setting is reflected in the menu, a dozen pages of heavy paper in earth colors, listing a dozen appetizers and nearly four dozen main courses, from enchiladas to mango duck. the menu is a tour guide, a dictionary and a survey of glamorized Mexican standards with a few surprises as well.

First come appetizers such as flour tortillas rolled around cheese and baked, taquitos, chile con queso, empanaditas and variations on a nacho theme. Second are the drinks -- those two together enough for a pleasant evening. Actually, it would not be a bad idea to stop after the first two pages. The margaritas are large and tart and strong, and the pina colada is too sweet for my taste but saved by fresh flavor. The sangria benefits from being drier than usual and is not only served in a brimming pitcher, but comes with the glasses already filled if, as the waitress put it, the bartender is in a good mood. That pitcher is $5.50, and the specialty drinks average $3. With a few of the appetizers, which average $2 to $3, one could end an evening with a flourish of a late supper for under $10 a person. The cheesy flour tortillas, called roscas, though once seasoned with zing, have lately been bland, but they are light and crunchy and go well with the fiery guacamole. Taquitos have also turned bland, but they are beautiful to behold, piped with sour cream and striped with guacamole and tomato-red sauce. Skip the empanaditas, tough little pastry triangles soaked in a strange cloying pineapple sauce.

If you go on to main dishes, walk a narrow course. The most familiar dishes -- the enchiladas, burros, tostadas and combinations thereof -- average $6 to $7. The more exotic and less strictly Mexican entrees -- garlic shrimp, chicken in tomatillo and chili sauce, spareribs, paella, veal and eggplant, fish and steaks -- average $8 and climb to $12.50. All come with the usual tortilla chips with salsa and soup or salad, the soup perhaps something as south-of-the-border as clam chowder (a passable version). Salad looks so pretty it leads you to expect too much, for in my experience it has been totally unseasoned greens with packaged croutons and chopped egg. Therein lies a theme, for the plates at Plata Grande do look lovely, being garnished with carrots and pinktinged turnips and radishes formed into flowers.

The cooking does not follow through. Dish after dish substitutes quantity for quality. The plates are enormous, and diners typically leave laden with leftovers. But those flower-decked mountains of food yield, in the mining, greasy, doughy, bland and heavy ore. On an early visit the shredded beef and the chicken that filled the enchiladas and tacos were fresh and spicy. Lately they have tasted as if the kitchen's guiding principle has been to avoid offending naive tastebuds. Even the first time around, the cook was pulling punches. Paella was well packed with seafood, but flavors were not well blended; the dish tasted as if it had been assembled after cooking. Spareribs had the steamy texture and flavor of reheated meat, with no character to the sauce and no exchange of flavors between sauce and meat. And duck was a soggy bird under a thick paste of assorted sweet and sour ingredients ground into a mishmash. The sauce could have been ground marmalade in brown sauce, though the menu described it as "marsala wine, mangos, mushrooms, brandy-soaked raisins, and marinated in port. Lavishly garnioshed with sesame seeds and served flambe." Actually, the duck was not flamed at all . . . the potatoes were flamed! Chicken Guadalajara seemed to have been reheated chicken with its onion-pimiento-olive-tomato topping added as a garnish; nothing intermingled.

One dish had substantial character, however. The chile verde, served either wrapped in an enchilada or ladled into a basket-shaped fried tortilla, was sparkling with fire, its green sauce pungent and tart as well as peppery with crunchy slivers of chile. But by my final visit, the meat appeared to have been cooked dry and added to the sauce; though the chunks of pork were plentiful, they had absorbed neither the moisture nor the flavor of the sauce.

In light of this, the refried beans and rosy-tinted rice were surprisingly good.

Plata Grande makes much of its desserts, listing them on a special menu ready for mailing to a friend. It boasts that all but the ice cream are homemade, but this is another of those disconcerting cases where supermarket cheesecake, carrot cake or chocolate cake might be better. The huge slabs of cake are damp and doughy, supersweet and missing any butteriness or richness. Sopapillas, however, were good on an early visit, particularly after having been slathered with honey-cinnamon butter.

Plata Grande has the potential of vitalizing Prince George's County night life. It looks beautiful and is more ambitious and sophisticated than all but a few of the county's restaurants, and it could be an excellent stopping place on I-95 between Baltimore and Washington. Unfortunately, the trappings don't really make the food taste any better than the fare of many a taco stand.