Major Plunkett has problems. Like Philoctete, he has a wound without purpose, one he didn't earn in his own name. He spends a great deal of time trying to understand what the insanity of war is all about and what purpose his own suffering can possibly have served. His participation in World War II furnishes him with plenty of fodder for PTSD-like symptoms and motivates his move to the island with his new bride, Maud.
At the heart of Plunkett's wartime memories are his comrades Tumbly and Scott, who die in horrible ways right before his eyes, moments before he is wounded by a concussive strike that scatters them. Good times. But the battles Plunkett fights are not confined to the front lines of the war—they're also fought back home, in the class warfare that seems even more inappropriate to soldiers who faced death on the battlefield, no matter their social standing.
Plunkett's retreat to St. Lucia was conceived of as a way to skip all of that and make a peaceful home for his wife. And son. Definitely for his son.
It's a little hard to gauge Major Plunkett's relationship with the Helens in this work. He clearly has love for the island as his place of refuge from the insanity of post-war Europe. It's also a place that gives him the opportunity to skip all the snobbery and elitism of his home country, for which he is eternally grateful.
But as a white British man on an island still dealing with the fallout of slavery and colonial occupation, Plunkett is in an awkward situation. He doesn't want to feel distant from his neighbors, and he certainly wishes they would stop thinking of him as a patron rather than a neighbor and fellow islander—but he also can't let go of his old military ways, barking commands rather than conversing with them as human beings. He's in a bit of a bind.
While the other St. Lucian's appear to have respect for him (remember that Hector calls him "honky" before he realizes who Plunkett is and then apologizes), it isn't until he loses Maud that he starts to reform his former stiff ways:
Calmly, and he began to speak to the workmen
not as boys who worked with him, till every name
somehow sounded different; when he thought of Helen
she was not a cause or a cloud, only a name for a local wonder. (LXI.iii.309)
The compassion he gets from Achille, Helen, Philoctete, and especially Ma Kilman as he recovers from the loss of his wife goes a long way toward closing the rift between them. When Ma, in her compassion, tells Plunkett that Maud is in heaven, "That moment bound him for good to another race" (LXI.i.307). Whereas Philoctete's pain isolates him, here we see it bridging space between the Major and the islanders.
It's not really until this moment that he gets over his obsessive, mythic view of Helen (the woman and the island) and feels his guilt assuaged enough to abandon the history project. He just doesn't need it anymore—he doesn't have to redeem Helen in any way, or pay homage to her beauty. He just needs to be a good person now, someone that Maud can be proud of in her green heaven.
We don't get an intimate look at Major Plunkett as a husband until after Maud dies. First, we see his grief upon finding her body on their bed, surrounded by his love letters to her. At this moment, we understand everything that Maud meant to him: "She was his orb and sceptre, the shire of his peace" (LII.i.261). Okay, okay—this is the language of youthful love and sentimental love letters, but there it is. He really was sweet on his girl, despite his earlier neglect.
He promised her, all those years ago, that they would "wait for marriage"—do things right, like an officer would—and they would have a son. They would definitely have a son. Oh but wait… Despite his noble actions and intentions, a child doesn't come to them. Ultimately, Major Plunkett has to concede that he really is the end of the line: There will be no more Plunketts to give their lives in the line of duty. The buck stops here.
That said, his discovery of the long-lost Midshipman Plunkett presents a unique, albeit kind of strange, opportunity to Major Plunkett late in life: He can finally have the boy he'd always wanted, and with a military career to boot. Finding the young man fires his imagination and changes the way he looks at the island:
On what hill did he pause to watch gulls follow a plough,
seabag on one shoulder, with his apple-cheeked sheen?
This was his search's end. He had come far enough
to find a namesake and a son. (XVII.iii.94)
It seems a little eccentric to adopt a dead man nearly two centuries older than you as a son. But remember: The poem is ultimately all about healing. Major Plunkett has a whole lot of healing to do in this work, and his dead ancestor fits right into the son-shaped hole in his heart.