MOM AND POP HAVE BECOME corporate giants. From a small beach-resort carryout shop they opened 29 years ago, Shirley and Brice Phillips have wrought six seafood restaurants. They even have their own seafood packing plants to supply their empire.

And now they have taken over the Flagship, which also grew from a homey waterfront seafood place to a 600-seat extravaganza with a 155-seat lounge. Whatever their secret, they certainly are successful: the Flagship is bustling. Although I am sometimes suspicious of having to wait in a cocktail lounge -- and thereby ring up a bar bill -- before tackling dinner, the tables at Phillips Flagship have seemed truly in demand when I visited.

So you start at the bar. In fact, the bar is a such an appealing place that I'd expect some people just stay put. There are piano music, a raw bar and an eclectic crowd. At happy hour the drinks are cheap, the spiced shrimp are good and the oysters and clams -- neither top-notch nor objectionable -- are 50 cents each.

When your name is called for a table, however, this well-oiled restaurant machine clanks. There's one bill for the raw bar, another for drinks, both to be settled before your table is given to somebody else. You arrive at your table feeling as if you have just run to catch a train.

The dining room space has been opened up to one long stretch of an indoor urban garden decked out for guests, with a party brightness as well as noisiness. And there is an outdoor deck that should be open by now, where I would opt to dine on a balmy day.

What Phillips does best is hire, encourage and train servers to be endearing. In the face of any crowd, they are cheerful; in light of long waits for food, they are able to make you feel cared for anyway. But the personality of Phillips Flagship stops at the kitchen door. The best you can hope for from this mass-feeder is fresh, plain seafood. Anything more than simple grilling or steaming is beyond its capabilities. You can get a good meal, but you need to walk a narrow line.

The menu is, expectedly, nearly all seafood; at lunch there are hamburgers and steak; at dinner, steak, fried chicken and ham. Otherwise, there are fish fillets broiled, baked, fried and grilled, three or four crab dishes, three shrimp dishes, lobsters broiled or steamed and fried scallops, oysters and clam strips. There are a daily special and seafood combinations, either fried or steamed. Despite the lengthy menu, though, the range is narrow. Fish fillets are limited to flounder, swordfish and a catch of the day -- shad with roe, on my last visit, although it ran out for a while during lunch. Clams are available on the half-shell or steamed, but the fried clams are frozen strips. In fact, the only dish more complicated than flounder baked with spinach was one day's scallops in mushroom cream sauce, but it was so dreadful -- bland scallops in a cream sauce that tasted as if it came from a test tube -- that I retreated to the plain stuff.

Phillips Flagship's proudest accomplishment, as far as I could tell, is its shad and roe. A large portion of perfectly fresh fish, broiled to a crusty surface, moist and supple, was paired with roe cooked to the same ideal state, topped with a couple of strips of bacon. There was no paprika acridity, nothing to sully it -- just good plain fish.

But such simple success is not cheap. The shad was $12.95 at lunch and dinner, with two vegetables or a salad. The vegetables can be good but the french fries are both frozen and greasy, and the tartar sauce is packaged in a little plastic cup. For comparison, at J.J. Mellon's restaurant, with more luxurious space and environment and more prompt service, impeccable shad without the roe but with excellent fresh green beans and saute'ed potatoes was only $9.50.

Nothing else I tried matched the shad. Stuffed flounder tasted cleanly fresh, but its flesh was a mite dry and the crab stuffing was insufficient as well as lacking succulence. Crab cakes had the good flavor of fresh crab and were light and crusty, but they were made from shredded crab rather than snowy lumps and bound with more creamy filler than is ideal; still, they tasted fine if not of first quality, and at $9.95 they were modestly priced. A steamed seafood platter looked impressive, though it is disconcerting to be presented a garish blue cafeteria tray for a $15 to $18 entree. And the single oyster didn't open, the couple of mussels were shriveled, the large shrimp were very salty, the clams were overcooked, the crab cluster had indiscernible taste and the lobster had a rubbery flavor. And it seems as if the seafoods are steamed in very salty water, perhaps in an attempt to intensify their flavor.

You will probably have had your appetizer while waiting for the table, but the menu of course lists raw clams and oysters, crab and shrimp cocktails, plus steamed shellfish. The hot appetizers include crab-stuffed mushrooms, decent grassy-green oysters rockefeller and odd bready clams casino. There are several seafood soups, with flavor that is weak but pleasant, and mushy-textured seafood. Sometimes, as with crab-asparagus soup, they sink to dreariness.

Desserts are sundaes and pies, probably the best of them the ice cream pies that appear as daily specials.

Phillips Flagship is clearly mass cooking for mass dining, but one of the few opportunities Washington offers for eating seafood outdoors on the waterfront. So if that is your aim, a simple fish fillet will realize it pleasantly.