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The beauty of a mom-and-pop eatery is that Mom and Pop tend to know their limits. Perhaps as a result, they often end up trying harder than the next guy: working round the clock, minding the tiniest of details in everything from the balance book to the contents of the fridge.

The original Sushi Sushi, in Bethesda, is testament to what can be accomplished by a restaurant of modest means—in effect, what can be accomplished when Mom and Pop eat, breathe, and sleep their business. Upon taking over the tiny space vacated by the Cafe Zino coffeehouse on Fairmont Avenue eight years ago, Shing-Fu Huang, the chef, and his wife, Ge Chen—regulars know to call her Lisa—immediately established themselves as a terrific option for moderately priced sushi. I considered the restaurant almost without peer in the category of workaday sushi, and it was a secret I guarded jealously, lest too many people find out about it and crowd out its charms.

The couple was, from the start, wise to keep things simple. The menu, even for sushi, was on the small side: no tempura, no grill stuff, not even the usual list of tricked-out specials. The benefit of such a limited focus, of course, is consistency. Tuna, salmon, and yellowtail were all reliable choices, and the last, at $3 for two pieces of nigiri, may well have been the best sushi value in the entire area. It was the little, distinguishing touches that kept me coming back: the smoky, barbecuey savor of the crispy broiled nigiri; the hand rolls, which, at their best, possessed a potato-chip-like crunch; the “crab” roll, which did a neat trick in turning the lowly crab stick into something surprisingly rich and sweet and buttery.

The couple’s success in Bethesda brought with it not just a bit of capital but also the confidence that comes of having pulled off something difficult. The result is a second location, on Macomb Street in Cleveland Park.

I would love to report that the new is every bit as good as the old, but unfortunately the very things that made the original such a gem are missing from the spinoff. Huang has assistants manning the sushi counter in Bethesda—Lisa still handles pretty much everything else—while he devotes himself to building the business at the much larger second location. In other words, Mom is in Maryland, and Pop has decamped for D.C.

It’s still too early to tell the full impact this expansion will have on the original, but on my last visit, the Bethesda location was beginning to show the first signs of slippage: several of the maki, too loosely wrapped, came undone before they could be dipped into the soy, and a sashimi of mackerel, once a reliable choice, was not just mealy but unpardonably fishy, too.

Meanwhile, the new location does not appear to be a spinoff so much as a more glamorized, more accessorized version of its unassuming sister. There’s a handsome, fully stocked bar in the back (the original serves only beer and warm sake), a Tara Thai–style paint job full of shimmering blues and greens (compare that with the nearly bare white walls in Bethesda) and a split-level design that allows for distinct areas within the restaurant. If you didn’t know the two restaurants were connected, you would never guess it.

There are menu changes, too. The roster of salads now includes the likes of a spicy tuna tataki—a complicated medley of chopped tuna, iceberg lettuce, pickled ginger, scallions, and sesame seeds in a ponzu sauce that only makes me pine for the simple, beautifully marinated octopus salad still available at the original but conspicuous by its absence at Cleveland Park. Two of the most obvious additions are also egregious missteps: a chicken-curry maki (the description alone is damning enough) and the Macomb Roll, which wraps a thin slice of cooked red snapper around a California roll—and demolishes, in a single swoop, pretty much every textural principle of good sushi. Instead of cool, delicate, slippery, and firm, the fish arrives warm and big and mushy and mealy.

The biggest of the changes, though, has to do with the prices, which remind me of the sort of startling inflationary jumps you tend to find when you compare the cost of movie tickets through the decades. A choice of two rolls costs $5.75 at the original; at the offshoot, at dinner, the same two rolls go for $8.50. Nowhere is this price hike more personally disconcerting than in the $4.50 nigiri of yellowtail. I can’t help it: The 50 percent markup takes just a bit of the pleasure away from what once felt like a real discovery.

The couple cites the cost of doing business in a high-rent neighborhood, as well as Huang’s desire to move beyond the simple roster of raw fish to include the sorts of menu items—tempura, for example—that were impossible in the Bethesda location. But what they have done, in effect, is to take a charming miniature and stretch it to fit a broader canvas. The new Sushi Sushi resembles about, oh, a dozen other places in the area in serving up decent, sometimes good raw fish in pleasant, comfortable surroundings.

The transition, Lisa tells me, has been personally wearying for the couple, who live in Rockville and who, having spent the previous eight years together every minute of every day, now see each other only at the end of the night.

“Late at night, ‘Hello, how are you,’” she says with a strained laugh, “then off to bed.”

Mom needs Pop: Their happiness, and ours as value-conscious sushi lovers, depends upon it.

Sushi Sushi. 4915 Fairmont Ave., Bethesda, (301) 654-9616; Sushi Sushi, 3714 Macomb St. NW, (202) 686-2015. —Todd Kliman

Eatery tips? Food pursuits? Send suggestions to hungry@washcp.com. Or call (202) 332-2100, x322.