Angie's is impossible to miss as you come over the bridge. They no longer sell gasoline (closest service station is just across from the Newbury Town Hall), but they still offer skilled and friendly service. They'll plow your driveway in the winter, too. Not only that, you get a nice thank-you card if you take your car there for repairs.
Click here to visit Angie's website.
Click here for a Google satellite image of Plum Island with Angie's highlighted.