Not the Plum Island of the mystery novel or the Animal Research Laboratory, this Plum Island is a small barrier island just off the extreme northeast coast of Massachusetts and is part of Newbury, Newburyport, Rowley, and Ipswich. The residential part is mainly Newbury and Newburyport. You can find some details and history on (of course) Wikipedia
The island is named for the hundreds of beach-plum bushes growing everywhere. (The harvest is unpredictable from year to year, but the small plums make great jam or jelly when you can get enough of them to bother with.) The clickable plum at the bottom of the page is from a bush on Old Point Road. It's about 2/3 actual size (depending on your monitor).
The island is separated - barely - from the mainland by the Parker River; the northern tip is at the mouth of the Merrimac River. Most of Plum Island (the southern part) is the Parker River National Wildlife Refuge, but the northern portion is densely settled with both year-round and seasonal residents. The architecture is an almost unbelievable jumble of old beach cottages, comfortable saltboxes, standard residential homes, intriguing modest little treasures, and trophy houses. The latter were historically on the oceanfront, but these are increasingly at risk to beach erosion, and this - along with creeping gentrification - has prompted recent outsize development on the marsh side. The gentrification includes changes - for better or for worse depending on how one sees or experiences the Island - in the 'downtown' with changes to the long-time icons Mr. Moe's and Dick's Variety, now The Cottage and Plum Island Provisions, respectively.
Virtually the entire oceanfront is beautiful public beach, although much of it is accessible only by foot. There are boardwalks in the Refuge that take you through the dunes and out onto the beach; these are great for sightseeing or birdwatching or just calming down.
The inland side faces spectacular tidal marshes (especially at sunset) with a continually changing population of birds ranging from plovers, redwing blackbirds and kildeer to egrets, gulls, great blue herons, and occasional snowy owls or ospreys. You can spot the marsh hawk
on her low-altitude hunting trips most mornings. Watch for the white spot on her tail. (Longtime bird-lister Janet Egan has passed the torch for Plum Island sightings to Tom Wetmore but she still has an active blog.)