There's something inherently mysterious about islands: inaccessible, self-contained places where the normal rules don't always apply.

Our own shores abound with examples. Think of Fishers Island, secretive enclave of the wealthy; Gardiners Island, frozen in time after nearly 400 years; or shape-shifting Sandy Point, slowly rolling ashore since the 1938 hurricane.

But for sheer atmosphere, there's no place quite like Plum Island. Named for an inoffensive fruit, it nonetheless exudes enigma and menace.

The island's forbidding aspect comes from the Plum Island Animal Disease Center, a federal laboratory that for years has been the subject of speculation, misinformation and fiction. The lab and the entire island are off-limits to pretty much everyone.

It's no surprise, then, that the island's history has been obscured by time. An impressive new book, "A World Unto Itself," reveals the story in full, and it contains a surprise or two.

Plum Island wasn't always mysterious, but it was always remote. Formed by a retreating glacier, the 3-mile-long land mass sits off Orient Point at the northeastern extreme of Long Island. It's visible in the distance from the southeastern Connecticut coast.

Never a heavily populated place, it was home to native Americans, then a scattering of English farmers. Until the 19th century, it was predictably drowsy.

Authors Ruth Ann Bramson, Geoffrey K. Fleming and Amy Kasuga Folk excel at placing the island within the larger sweep of history, and early on this helps the narrative because not much was happening there.

The fact that the British fleet anchored off the island after Benedict Arnold's invasion of New London in 1781 is pretext enough for the book to recount that entire adventure. The tale is well told but doesn't feel especially relevant. In the War of 1812, at least, some of the action actually occurred on the island.

The meat of the island's history revolves around three things. First is its lighthouse, built to aid mariners in the dangerous waters of Plum Gut, off Orient Point. The first light there was built in 1827 and was replaced by the current house in 1869.

The story of the lighthouse is that of its keepers, a succession of men who lived in isolation with their families to maintain the lamp and assist stranded sailors. Many left detailed records of their days, and the book paints a lively picture of guests from passing ships being entertained with fishing and hunting expeditions.

For a while Plum Island was a popular vacation spot, and some keepers made extra cash renting space to tourists who wished to stay longer. The federal government frowned on this as a distraction.

In 1885 workers were sent there without warning to tear down the assistant keeper's house, which was also a makeshift hotel. The tourist trade slowed to a trickle.

Plum Island's days as a place of leisure were brought to an abrupt end by an unlikely cause: military fears that New York City was vulnerable to enemy attack.

One of the book's most interesting sections lays out the little-known story of how a chain of coastal defense fortifications were built on islands off the southeastern Connecticut coast, starting in 1897.

The idea was that a naval assault on New York would probably be made via Long Island Sound, which was undefended. A string of five forts were planned to close up the Sound's eastern entrance from Orient Point all the way to Watch Hill.

The largest of these, Fort Terry, took up the entirety of Plum Island, which the government bought and put off-limits. The fort, active through World War II, is the second major aspect of the island's history.

The chapters on the fort contain plenty for military buffs about artillery powerful enough to shatter distant windows. But they also feature the book's principal human drama: the 1914 court-martial of the commanding officer on trumped-up charges of homosexuality.

Maj. Benjamin M. Koehler was a rising star in the Army who brought needed discipline to Fort Terry, where a freewheeling culture of alcohol, gambling and sex had taken hold, the book says.

That bred resentment among a few of his subordinates, and soon accusations surfaced that Koehler had touched various men inappropriately. The incidents supposedly occurred in full view of others, but no witnesses could be found to corroborate the claims.

Still, the Army brass seemed eager to make an example of him at a time when homosexuality was illegal in the civilian world but unaddressed by military law. He was tried on the island, convicted and dismissed from the service.

The scholarship it took to reconstruct this obscure episode is hinted at by 139 footnotes that fill more than six pages. Prodigious research and smooth, readable prose are hallmarks of the entire work, as are a generous helping of photographs.

Eight appendixes give encyclopedic detail on such arcana as individual buildings at Fort Terry, myths and legends of the island and shipwrecks in local waters.

The third major part of Plum Island's history began when Fort Terry was rendered obsolete after World War II. The island was to be sold as surplus government property, and competing interests vied to make it either an expensive resort or a low-key recreational facility.

But it turned out the government had no intention of selling and had been quietly working to put either the Army Chemical Corps or a Department of Agriculture disease laboratory there. When this became public, outrage erupted, and the seeds were planted for the longstanding perception of the island as a place of intrigue.

The Army Chemical Corps stayed only briefly, and its work there has never been declassified, the book says. The animal disease lab opened in 1956 and was primarily concerned with foot-and-mouth disease, which affects cattle and swine.

Despite many scientific achievements at the lab, public suspicion has been stoked by incidents such as a security lapse that allowed a virus to reach an animal holding area, leading to the slaughter of 200 cattle, pigs, sheep and ponies. A novel by Nelson DeMille fed on existing fears.

The lab is slated to close in the next few years, and a new chapter in Plum Island's history will be written. But what it will entail is, like everything else there, a mystery.

j.ruddy@theday.com