The story is set on the Great Marsh, a desolate expanse of grass, reeds, and tidal pools on the Essex coast of England, between the village of Chelmbury and the ancient Saxon oyster-fishing hamlet of Wickaeldroth. Along the River Aelder runs an old sea wall, at whose corner stands the ruin of an abandoned lighthouse, long since breached by the sea. The narrator introduces the story's two central figures, a lonely man with physical disabilities and a child who learns to see past his appearance, and notes that the tale has been pieced together from fragments and many sources.
In the late spring of 1930, Philip Rhayader, a 27-year-old painter of birds and nature, purchases the abandoned lighthouse and the surrounding marshland. He has a hunched back and a left arm bent at the wrist like a bird's claw, and the villagers of Chelmbury regard him with suspicion. Despite his disabilities, Rhayader loves humanity deeply, but his appearance repels others, and years of painful rebuffs drive him into seclusion. In his retreat, he devotes himself to painting, sailing a 16-foot boat with remarkable skill, and building a bird sanctuary around the lighthouse. He keeps pinioned geese and ducks, birds whose wings are clipped to prevent flight, whose presence signals safety to wild migrants. Hundreds of wildfowl winter with him each year, and their faithful return gives him happiness and purpose.
One November afternoon, three years after Rhayader's arrival, a frightened girl of about 12 named Frith (short for Fritha) approaches the lighthouse carrying a large wounded white bird stained with blood. She lives with the fisherfolk at Wickaeldroth and is terrified of Rhayader, whose appearance has spawned dark legends among local wildfowlers, but she has heard he can heal injured creatures, and her compassion for the bird overcomes her fear. She nearly flees at the sight of him, but his deep, kind voice steadies her.
Rhayader identifies the bird as a snow goose from Canada, marveling at its unlikely presence in England. He splints its broken leg, bandages the wing damaged by gunshot, and tells Frith how the young bird was blown off course by a great storm during its southward migration. He names it "La Princesse Perdue," the Lost Princess. Frith laughs with delight when the goose nibbles grain from Rhayader's hand but then flees in alarm. Rhayader asks if she will return to check on the Princess, and she calls back her agreement.
The snow goose recovers, and Frith becomes a frequent visitor through the winter, her fear replaced by fascination with the bird and the world Rhayader opens to her. One June morning, the snow goose joins a departing flock of pink-footed geese and flies away. With its departure, Frith stops visiting, and Rhayader experiences profound loneliness. That summer, from memory, he paints Frith as she first appeared: a child carrying a wounded bird through a November storm. In mid-October, the snow goose returns, and Rhayader weeps with joy, sending a message to Frith through the Chelmbury postmistress. Three days later, she comes shyly to visit.
Years pass on the Great Marsh, and the snow goose governs the rhythm of Frith and Rhayader's relationship. When the bird is present, Frith visits regularly; she and Rhayader sail together, tend the sanctuary, and she learns bird lore from him. When the snow goose departs for the summer, an unspoken barrier rises between them, and Frith does not come. One year the bird fails to return, and Rhayader is heartbroken. But in the fall it reappears, and Rhayader realizes with a shock that Frith is no longer a child.
In the spring of 1940, as World War II escalates and bombers frighten the birds into early migration, Frith and Rhayader stand on the sea wall watching the last unpinioned, or free-flying, geese depart. The snow goose takes flight but circles the lighthouse twice before dropping back to earth, choosing to stay. Rhayader declares that the Lost Princess is lost no more and has chosen this as her home of her own free will. Frith senses the longing he cannot express because of what he feels himself to be, and is frightened by the intensity. She says she is glad the Princess will stay so he will not be so alone, then walks swiftly away. Far down the sea wall, she pauses and feels a sharp sense of loss.
Three weeks later, at the end of May, Frith returns to find Rhayader loading his sailboat with supplies. He explains that he must sail to Dunkirk, where a British army is trapped on the beaches and every available vessel is being called to help evacuate them. Frith senses mortal danger and begs him not to go. Rhayader explains that the soldiers are like hunted birds, storm-driven like the Princesse Perdue Frith once found wounded in the marshes. For once, he says, he can be a man and play his part. Frith, seeing him transformed, offers to come, but he refuses: Her place would mean one fewer soldier saved. He departs in the night, and as Frith watches, the snow goose sweeps past and flies above his boat in slow, wide circles. "Watch o'er him. Watch o'er him" (40), she whispers.
The narrative shifts to fragmentary witness accounts from Dunkirk. Out of the smoke and bombardment appears a small sailboat navigated by a bearded man with a hunched back and a bent hand, a rope clenched in his teeth, while a white goose circles overhead. He ferries soldiers to offshore vessels all afternoon and through the night, helping evacuate over 700 men without losing one. When the last man is off, Rhayader sails back toward Dunkirk with the white bird still circling his boat. A retired naval officer later recounts finding a derelict sailboat with a dead man and a goose perched on the rail. The goose's presence draws the crew off course, saving them from a floating German mine. The mine's detonation sinks the small boat, taking Rhayader's body down with it. The goose rises, circles three times as though saluting, then flies west.
Frith remains alone at the lighthouse, tending the pinioned birds. She discovers the painting Rhayader made of her years earlier and is deeply moved. Long before the snow goose returns, she knows from instinct that Rhayader will not come back. One sunset, the snow goose plummets from the sky, and the sight releases the truth of her love in a flood of tears. Her heart answers: "Philip, I love 'ee" (56). The snow goose skims low over the enclosure but does not land. It soars in a wide spiral, then climbs. Frith sees no longer the bird but the soul of Rhayader taking its final farewell. She stretches her arms skyward, crying out her godspeed, and watches until the goose vanishes. Then she takes the painting, hugs it to her breast, and walks home along the old sea wall.
For weeks, Frith comes each night to feed the birds. Then one morning a German pilot mistakes the lighthouse for a military target and destroys it. That evening, the sea has moved in and covered everything. Nothing remains. No marsh fowl have dared return. Only the gulls wheel and cry their lament over the place where the lighthouse once stood.