Charles Wallace awakens on the star-watching rock with Gaudior, who explains the wind pulled him from Chuck, and that Mortmain had the boy institutionalized. Gaudior urges haste, and they fly through time without Charles Wallace secured. They experience a penetrating spiritual and physical cold, then land in a Projection: a ruined, desolate city of cracked cement and tall windowless buildings where two gas-masked guards raise machine guns at them. Gaudior escapes with Charles Wallace, uncertain whether the guards could have harmed them. They land in springtime, and Gaudior grows anxious.

Meg kythes a vision of the star-watching rock in 1865, where a young woman sits with a young man who seems somehow wrong. Charles Wallace endures an agonizing Within-ing, feeling torn between opposing forces as he is forced into the body of Matthew Maddox, whose legs are useless after his accident. Matthew meets Zillah Llawcae at the rock. Matthew explains he asked Jack O’Keefe, the hired hand, to deliver his note to her. Matthew reveals his twin brother Bran is in pain and danger. He explains that five years ago his horse crashed into a fence and rolled over him, crushing his pelvis and legs and fracturing his spine. Bran had refused to pity him, instead demanding independence. Matthew lived vicariously through Bran until the war separated them.

A week later, Mr. Maddox announces that Bran has been wounded by a bullet in the leg and is returning home. Bran arrives limping and withdrawn, shutting Matthew out telepathically. After three months, nothing has changed. Zillah reveals Bran asked her to return his ring, but she refuses. Matthew tells her he saw Gwen kissing Jack, whom he distrusts for his cruelty after witnessing him kill a puppy. He urges Bran not to abandon Zillah. Eventually, Bran breaks down, confessing the war’s horrors: witnessing brother kill brother, seeing God withdraw from the battlefield. Matthew comforts him, and their connection is restored.

Bran announces he will join the Welsh colony in Vespugia, Patagonia, founded by their distant cousin Michael Jones and landowner Love Jones Parry. He recites a poem about Madoc, who fled Wales when brother fought brother, comparing it to the Civil War. Zillah declares she will marry Bran and accompany him, but her father, Dr. Llawcae, forbids it, insisting she wait until she is 18. When Mr. Maddox catches Gwen kissing Jack, he orders Gwen to accompany Bran instead.

After their departure, Matthew’s writing career flourishes. Letters arrive from Bran describing the colony, including Rich Llawcae, who loves Gwen, and Gedder, an Indigenous man who assists the colony but later seeks power and influence. Gedder pushes his sister Zillie—who resembles Zillah and Gwen—toward Bran. Matthew’s health fails, and in fevered dreams he encounters a strange boy whose dream merges with his. The boy warns him to stop Gedder and protect the bloodline. Matthew accuses Dr. Llawcae of selfishness for keeping Zillah tied to him instead of letting her go to Bran and, weeks later, secretly gives Zillah money from his earnings as a writer and a steamship ticket, urging her to go because she will tip the balance. She agrees and departs in secret.

Matthew finishes his novel about the primordial pattern of fratricide. Through kything, he witnesses events in Vespugia: Gedder and Rich quarrel on a cliff’s edge over Gwen. Rich disarms Gedder’s knife, and Gedder falls to his death trying to retrieve it. A final letter from Bran confirms Zillah’s arrival and Gedder’s accidental death. Rich is bringing the homesick Gwen home, and the store will become Maddox and Llawcae. The letter ends with a prophecy that Blue will right the ancient wrong.

Matthew is seized by a fatal coughing fit as darkness and a putrid stench overtake him. In the present, Meg wakes calling Matthew’s name. Unable to sense Charles Wallace or Gaudior, she goes downstairs and asks Mrs. O’Keefe if Zillah reached Vespugia. Mrs. O’Keefe says she forgets. Meg insists that there was a time when the past had not yet happened and that Charles Wallace may have changed the Might-Have-Been. Mrs. O’Keefe suddenly stands and demands they take her to Chuck before it is too late.