The Plum Island Grille is open for dinner and limited lunch at various times depending on the season; check their Facebook page for schedules and menus.
Just over the bridge.
Nice menu, very friendly service, sensible portions, warm atmosphere, tiny bars inside and outside (and usually engaging conversations with neighbors if you eat at the bar). Beer and wine and interesting martinis. They do need, though, to learn how to serve oysters; not always a great choice of them, and rarely on a bed of ice.
Not quite as special as it once was since it has become a destination (although surely good for business!), but still a great experience for drinks or dinner at sunset - especially at the outside room.
Click here for a Google satellite image of Plum Island with The Grille highlighted.