Lyman Ward is a wheelchair-bound ex-historian who's diving into the most ambitious project of his life: writing a partially fictionalized account of his grandparents' lives. In order to do that, he plans on holing up in their old home—dubbed the Zodiac Cottage—and writing all day, every day.
His grandparents are Susan and Oliver Ward. Susan comes from Milton, New York, where she was heavily involved in the literary scene with her best friends, Augusta and Thomas Hudson. Susan and Oliver meet randomly one New Year's Eve and start exchanging letters soon afterward. In about five years, they're married, and Susan moves out west to join Oliver, who works as a mine engineer.
They move to a small mining camp called New Almaden. Susan actually digs it there, but Oliver eventually gets into a beef with his boss, which leads him to abruptly resign from his position. The Wards spend the next several years trying to figure things out (Oliver even tries to start a cement business), but Oliver is eventually forced to take another mine job in Leadville, Colorado.
Susan actually loves Leadville. There, Oliver has an assistant named Frank who seems to like Susan an awful lot, too. Like a lot, if you catch our drift. Although Leadville is a boomtown when they arrive, the town eventually slows down, and the Wards are forced to move once again. They spend a month or two in Mexico while Oliver inspects a mine, but that job ends rather quickly when the mine in question proves to be worthless.
After several more years of planning, the family moves to Idaho so Oliver can work on his latest business venture: an ambitious plan to irrigate the desert. Frank even travels to Idaho to work on the project, too, which Susan is a little too excited about. (By the way, the Wards have three kids at this point: Ollie, Betsy, and Agnes.) The family lives in a tiny shack for several years before financial problems force Susan and the kids to move back east.
The project eventually gets more financial backing, and Susan moves back to Idaho—though this time, they live in a swanky McMansion. Susan and Oliver's marriage has grown really tense over the years, however, so Susan finally gives in and starts an affair with Frank. Ultimately, this affair takes a turn for the tragic when Susan's youngest daughter Agnes drowns while accompanying Susan on one of her trysts, and Frank commits suicide several days later. Oliver never forgives Susan for this.
Back in the present, Lyman is horrified by this revelation. He's actually been dealing lately with his own marriage problems: his wife, Ellen, left him for his surgeon while he was receiving treatment for a bone disease. After having a trippy dream in which Ellen comes and forces him to give her a tour of the Zodiac Cottage, Lyman realizes that he should be the one to make the first move and forgive Ellen for her betrayal. After all, he doesn't want to make the same mistake that his grandfather did by never forgiving Susan.