All life is a struggle against entropy, but the people in this novel are acutely aware of the encroaching decay, internal and external. “She was coughing,” the story opens, “always coughing, and sometimes she coughed up blood.” That’s Marantha Waters, who has just sailed to San Miguel for “the virginal air” that will make her well again. A ridiculous proposition, of course, but her new husband, Will, is so excited about raising 4,000 sheep on this remote and treeless rock that everyone’s pretending things will work out fine.

Using their last $10,000, they’ve brought their teenage daughter, Edith, and a serving girl named Ida to “the Graveyard of the ­Pacific.” Marantha knows that their house — the only structure on the 14-square-mile ­island — won’t be anything special, but she’s shocked to find a sheep-scented shack: “everything damp, always damp, mold creeping over the mattress like a wet licking tongue and the walls beaded with condensation” — just the first of many bitter disappointments Marantha will endure over the coming months as her lungs noisily rot.

Early one dark evening when young Edith alludes to “Wuthering Heights,” she and Ida laugh, but the spores of gothic tragedy are already lodged in this house. As the days fall away “like the skin of a rotten fruit,” Boyle carefully records the strained breath of a dying woman trying to combat the twin infections of tuberculous and resentment, determined to support her husband in a plan that cannot work. “She didn’t want to spoil things for Will,” Boyle writes. “This was his idea, his venture, his dream, and he’d talked it up so many times over the past months it had become a litany of success and increase and health abounding.” But deep within herself, deeper even than the mycobacteria she can’t cough away, she has lost the habit of hope.

This is a sensitive portrayal of a marriage stressed by the toxic intermingling of illness and naivete — just the right conditions for a host of more pernicious pathogens to breed and transform both partners. We all imagine that someday we’ll be stoic patients or patient nurses, but Will and Marantha have positioned themselves to confront this challenge in a lonely place where everything is “smeared with mud and the very walls reeking of mold and rot and the sort of deep penetrating dampness no stove could ever hope to dry out.” It sounds grim, I know, but the intensity of Boyle’s narrative never lets it flag, and soon enough a story of decline becomes a desperate story of escape.

How striking it feels, then, when the second half of the novel opens decades later in comparatively sunny 1930. Another optimistic couple arrives on the island. The parallels are subtle and unforced. The eager husband, Herbie Lester, is a war veteran like his predecessor, though of World War I instead of the Civil War. Once again, wool from island sheep will weave their fortune. But this time, the wife, Elise, is just as enthusiastic as her husband. After the Waterses’ dismal decline, Herbie and Elise sound like Adam and Eve before the Fall: “That first week was an idyll, the two of them alone in an untamed place and nothing in the world to intrude on the slow unfolding of a peace and happiness so vast she couldn’t put a name to it.”