Open Sunday through Thursday, 11:30 a.m. to 3 p.m. and 5:30 p.m. to 2 a.m.; Friday, 11:30 a.m. to 3 p.m. and 5:30 p.m. to 3 a.m.; Saturday, 5:30 p.m. to 3 a.m. AE, MC, V. Reservations. Prices: Main courses average $7 at dinner: after-theater menu ranges from $2.50 to $6 for light dishes and platters. Full dinners with wine, taxes and tip average $20 to $25 a person.

Someone asked where to find a restaurant, not too expensive, with a medieval theme for the brirthday of a 12-year-old is a Dungeons and Dragons fanatic. Le Cellier des Moines made it a happy dinner party. Someone needed a place to eat after the theater, with a choice of light snacks or a or a full meal. Le Cellier des Moines serves a post-theater menu from 11 p.m., ranging from onion soup or pate to beef bourguignon or guiche, with cold plates of seafoods, meats or cheeses included in the choices. And yet was looking for dinner with entertainment. Le Cellier des Moines employs a folk guitarist in the evening.

Le Cellier des Moines would be the best-hidden Georgetown restaurant were it not for the brown-robed monkish doorman who ushers you from M Street down the sadly barren corridor to the downstairs dining room. The theme is both monkish and puckish, for at the entrance to the cellar is a confessional inhabited by a cardboard friar, and all the waiters are in monks' robes. The theme is not carried further -- no Gregorian chants, no waffers. The lugubrious medieval prints on the walls are softened by the rows of wine bottles used as architectural ornaments, the booths and walls lined in brick -- fake -- and the garish red and green lighted sculpture identified in changing colors as Green Fire. The room is dim, with pockets of seclusion in the booths, but nevetheless noisy.

The monk who takes your order is likely to be a jovial one, but sometimes seemingly preoccupied with higher thoughts, for dinner can come slowly. Single diners, however, and they are frequent, are served with dispatch.

Encouragement of frivolity starts with the wine list, reasonable enough to tempt the recalcitrant. The choices are modest, but so are the prices. Most of the wines, new vintages as one might guess, are under $10.

One of the strongest assests of Le Cellier des Moines is its prices; few Georgetown restaurants have a choice of such appealing main dishes at $7. One night that included softsell crabs; trout with capers, lemon and croutons; roast beef bordelaise; roast lamb with madeira sauce; and four more for $7.25 as well as two for $6.25. Even more reasonable is the late-night menu (from 11 p.m.), which charges only $4.25 for beef bourguignon, $2.50 for a cheese plate and around $4 for quiche, cold cut platers or steak tartare.

Price would be irrelevant, were it not for the quality. The menu and the waiters stress that the fish are fresh. For the most part, they are, as is the duck; in this city, a duck that hasn't been frozen is a rarity. The menu also breaks through standard French restaurants boredom on occasion. Specials often include pork dishes, a welcome addition. Filet of trout is sauced with lime and vermouth, filet of sole with cognac and cranberries.

So the intentions are good. But, as with other roads paved with good intentions, there are some devilish outcomes. Take that fresh duck, for instance. It is meaty and true in flavor, not crisp but nevertheless nicely cooked. And it is garnished with delicious glazed fresh pineapple slices. But its sauce is syrupy and excessively sweetned, no complement to the dish. Ask for the sauce on the side or left in the kitchen.

Sauces gave me a problem on another dish as well. The daily special was trout grenobloise, with capers and croutons. That's a savory recipe, but I wanted to try the trout with lime, vermouth and cream. The wrong dish came, the trout grenobloise, which was graciously returned to the kitchen. Soon, too soon, came an identical trout, this one stripped of skin and sauced with a lovely fragrant, tart cream sauce. In it were floating capers, though the waiter affirmed that capers are not generally in the dish.It was a very good fish, fresh and delicate and beautifully boned. And the capers did it no harm. But I wish the kitchen had been a little more subtle in its recycling.

I also wish the kitchen would do the same in regard to its spinach. Spinach often accompanies the main dishes, and it is creditable cooked leaf spinach. The salade des moines is a spinach salad with bacon, walnuts, raw mushrooms, hard cooked eggs and a good, zesty mustard sauce. It is a lively combination, marred only by being served in a bowl too small to toss the dressing. Then spinach is offered as a soup, but a dull, watery dark green puree that tastes as if its only purpose was to use up the leftover spinach.

The chef has an admirable touch with seasonings. In previous visits I have tried pork with mustard sauce and chicken with vinegar that were sprightly. Outstanding would be praise too grand for them, but they were certainly good food for the money. For dessert, too, I have tried le mystere des moines, a fine frozen bombe, and a chocolate pot de creme that was grainy but deliciously bittersweet. Both, however, were sullied by inferior whipped fluff on top.

If the execution of the dishes is inconsistent, the ideas are fresh. A late supper for two that is light and worthy of lingering is a seafood platter, $12, consisting of a half dozen each of oysters, clams, boiled shrimp and crab claws, with a small mound of tender scallops, all bedded on ice, with cocktail and tartar sauces (plus mignonette if you ask). The clams and scallops were the best of the lot, but even with the lesser seafoods, the nibbling is a pleasant activity to accompany a debriefing after the threater or movies. The late night menu includes, as well, a few appetizers, onion soup and desserts.

Le Cellier des Moines gives you advance notice of what to expect. The daily specials are posted at the street entrance, and the prices should indicate that moderate expectations are in order. For the price, though, you get a broad choice, an entertainment bonus, and pleasant culinary surprises here and there. Le Cellier des pmonies is no penance.