That secret little seafood restaurant, the one with terrific fresh fish at bargain prices, the one so hidden in an alley that nobody will find it without being told about it, the one that you dreamed up in your restaurant fantasies -- it's come true and it's called Captain Days.

It really is in an alley, between 18th and 19th streets north of M Street, adjacent to Mr. Days, which obviously fathered it.

It really is a bargain, with lobster well under $10, the ultra-fashionable blackened redfish at $7, and mesquite- grilled red snapper at $6.50. You could have clams on the half-shell and a lobster-scallop pan roast for under $10.

It really does have fresh fish, a changing variety of daily catches, and it has a chef who treats them with respect.

Captain Days is a casual place, long and narrow with one wall taken up by a bar for eating as well as drinking. Although it looks the kind of place that you'd expect to have indifferent food, the lobster tank at the entrance suggests a certain respect for freshness and the grill and stove behind the bar look well attended by the chef. But don't expect tablecloths more fancy than vinyl, and the paper napkins aren't even heavy-duty ones. The brick walls are decorated with fake game fish, and the room is as noisy and clattery as the alley outside. Captain Days can get crowded at lunch, and often could benefit from another waiter or two, and certainly from one who had a greater sense of order and timing. The waiters and bartender are friendly sorts, which sometimes counts more than efficiency if you're not in a hurry.

What Captain Days really has going for it is good food. You can choose from specials on the blackboard, but the stuff on the menu is just as good -- soups, raw bar standards and barbecued shrimp for starters, seafood salads, sandwiches and entrees that include fried seafoods, steamed lobsters, baked stuffed flounder and a half-dozen kinds of fish grilled over mesquite. The only meats on the menu are chicken salad and barbecued chicken.

The soups are clearly homemade and of good ingredients, though tending toward blandness. Clam chowder is thick with hunks of clam, which are a bit mushy but fresh or frozen, not canned; and it is not mucked up with anything but cream and potatoes -- good soup. A seafood gumbo one day was a vegetable soup with fresh okra that was not overcooked, with chunks of fresh fish. And a seafood chowder another day was timid but again made from fresh ingredients. Clams or oysters on the half-shell are a mere $3.50, but the best of the appetizers is barbecued shrimp, five of them wrapped in bacon, basted with a fairly hot tomato-based sauce and grilled on mesquite.

Mesquite and Cajun cooking -- this menu could hardly be more fashionable. But that doesn't detract from the quality. The blackened redfish is not quite as charred and crusty as the correct version, but it tastes awfully good -- spicy but not so hot it sears -- and juicy, except in the thinner parts of the fillet. Mesquite-grilled swordfish came as a large slab about a half-inch thick, slightly crusty and a bit smoky from the grill, cooked just right. As for the fried seafoods, the crab cakes are light enough and not greasy, but lacked flavor when I tried them; shrimp, too, were lightly breaded and cooked just to crispness without greasiness, but flavor was lacking. Soft-shell crabs had more intrinsic sweetness to them. In general, though, I would concentrate on the grilled fish. The pan roasts are said to be a house specialty, but they are one I don't really understand. They taste like a big bowl of thick, slightly tomatoey, worcestershire-sharpened and faintly sweet cream with some nice seafood floating in it. They would be delicious as a cupful, but are overwhelmingly rich as a bowlful. The seafood salad, when I tried it, tasted as if it had hung around too long and the seafoods had grown too firm and too tart from the marinade.

The accompanimentsy are almost delicious. The french fries are fresh shoestrings with the skin on, but they have been consistently undercooked so the crunch is in the flesh rather than the crust. And the coleslaw is sprightly in texture but tastes of hardly any dressing or seasoning. Fresh vegetables are good, another nice surprise in such a casual kind of restaurant.

Daily specials range far, from cold pickled salmon to lobster fra diavolo to shrimp and scallops mornay to sea trout with louisiana sauce. This is the kind of restaurant that tempts you to see what the cook has in mind today. And some days there are lobster bargains, so it is worth checking the signs.

As for dessert, the only one made in-house, we were told, was a chocolate whipped cream roll. It was just that, a straightforward rendition, and pretty good except that it, too, tasted as if it had spent too long in the refrigerator. Besides, portions are so large that you will probably find a mug of coffee sufficient ending for a good fish dinner.

Captain Days' location suggests that lunch is its prime time. And its prices encourage dinnertime luxury at midday. Elsewhere downtown you would pay almost as much for a hamburger as you would for a lobster you picked live from Captain Days' tank.