It’s hard to miss Koi Sushi Lounge. Even if the GPS on your cellphone were to die en route, you couldn’t pass this former diner sizzling with light at one end of Sayville’s otherwise demure Main Street.

The blingy facade portends what waits inside. In the foyer, a Plexiglas sculpture pushes neon bubbles upward. A tiny wind chime trills guests’ entrances into the dining room, still lined with booths as it was during the 75 years that the Sayville Modern Diner operated here. Yet most other vestiges of that business, which closed in 2015, look as if a disco were trying to invade: The lacquered bar is beset by neon lights, the bathrooms are behind a beaded curtain, and the backlit sushi counter is Jetsons-futuristic.

The curls of raw fish over rice that sushi chef Danny Dong sends away from that counter come on plates decked out in rococo details — droplets of bright sauce, flower petals, fans of upright bamboo.

The transformation from diner to funky sushi lounge is the work of the Ren family, a restaurant-centric clan that leased the space two years ago. Darren Ren, Koi’s general manager, said his family — led by cousin Coco Ren — opened Koi to address a gap in the downtown Sayville sushi scene (though there are a few other sushi places nearby). Koi’s menu is a multipage affair that also includes a few Indonesian, Thai and Chinese dishes, many cooked by chef Jack Ren, Darren’s uncle.

It makes choosing from the menu feel like, well, choosing from a diner menu. Fortunately, the servers will gently guide you — away from too much food, for instance, or dishes that are too similar.

It’s smart to stick with the main event, Japanese plates, as the fish and meats are of prime quality and the kitchen is deft with sauces. Paper-thin slices of seared filet mignon (called carpaccio on the menu, but more akin to tataki) are delivered in a pool of barely spicy yuzu sauce, velvety and almost slurpable. Ditto for butterlike morsels of white tuna anointed with crisped mushrooms and truffle oil in a slick of citrusy sauce. Eclipsing both are rolled cigars of yellowtail, also in wasabi-laced yuzu sauce, topped with daubs of sour plum sauce and crispy garlic slices that lend crunch and punch.

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Koi’s menu practically sags under the weight of dozens of rolls. There’s a roll topped with three kinds of caviar (the Crystal Roll), a roll stuffed with lobster and topped by seared steak (the forgettable Giants Roll), and a roll built with crunchy lobster tempura and slices of mango (the tastier Bomb Lobster Roll).

No doubt, diners will order plenty of these pretty, vivid bundles — even if, with eyes closed, one may be barely distinguishable from the next. But to pass over Koi’s nigiri sushi, from fish procured at a Queens fish market and by mail order from Japan, is to miss out. Dong is a 25-year veteran of Long Island’s sushi scene, including Kaji in Garden City, and he has mad knife skills. The ends of each jewel-like cut of sushi languidly graze the plate; they’re neither too cool nor too warm, and the sticky balls of rice beneath each are well-seasoned. Chef is generous, too: A regular sashimi plate boasts 15 hunks of bouncy fish with little adornment other than lemon wedges, wasabi, and . . . orchids.

For all of the ornate presentation, the kitchen has an unfortunate crush on orchids, orchid petals and plastic flowers. The first one is charming; the third or fourth becomes distracting. It’s a minor gripe. More challenging to fix might be some of the culinary misfires: rubbery grilled squid; an overdressed, limp kani salad; too-oily udon; sautéed oysters suffering with fishiness.

But all-that-is-not-sushi can occasionally work, too: Soups are on point, from a mushroom-studded clear soup to a tom-yum soup dense with scallops and snappy shrimp. Tempura is crisp and capable, and pork katsu is hefty enough for the most dedicated carnivore.

Diners can chase it with one of the excellent sakes, or a curiously Western-style dessert such as Tuxedo Bombe. Or, both. This is fusion, after all — exuberant, playful and satisfying.