Exposition (Initial Situation): Things Fall Apart
Life's not looking too good for Codi Noline. She's a med school dropout who works at a 7-Eleven; she's broken up with her boyfriend; her estranged father has Alzheimer's; and her sister and Hallie has just left the country to go plant cotton in war-torn Nicaragua. Codi goes to Grace, Arizona, the town where she grew up, to try and take care of her dad. It's just another stop on that old highway of life.
Rising Action (Conflict, Complication): Girl Meets Dog
Weirdly, things aren't actually that bad in Grace. Codi's living with her best friend, dating her own former high school boyfriend, and teaching high school biology—which is actually kind of fun. It's too bad Codi totally hates Grace, and Grace totally hates Codi, or she might be able to settle in there.
The only other problem is that Grace is doomed: the mining company that owns basically all of the land and water around has been poisoning the river with sulfuric acid. On top of that, the company wants to divert the river altogether, so basically Grace has about three years of existence left.
Climax (Crisis, Turning Point): Making History
The climax begins when things in Grace seem, terrifyingly, to be working out. Codi starts remembering parts of her childhood. Her relationship with Emelina is super fulfilling, and she and Loyd are falling in love. The Stitch and B**** ladies have a plan to save Grace by selling peacock piñatas, and it's actually working. In spite of all of this, Codi is still planning to leave in a few months to go and hang out in Colorado with her ex-boyfriend, Carlo. When Doc and Codi learn that Hallie has been kidnapped in Nicaragua, Codi really starts to fall apart.
Falling Action: Collective Memory
Out of guilt, Codi's leaving in just a month or so, and she dumps Loyd. She still goes around his place for a little bit of that train-driver magic on the low, but for the most part, she spends her time writing to everyone she can think of with pleas to save Hallie.
While that's happening, the women's plan to save their town starts to bear fruit. Their piñatas have received some coverage, and an art collector has come to town and shown the ladies of Grace how to get their town on the historical registry. Meanwhile, it's becoming clear to Codi that she and her family are not, in fact, outsiders in Grace. There seem to be super old graves in the graveyard with Doc Homer's name on them.
Resolution (Denouement): There's No Place Like Home
The plot begins to resolve when Codi and Doc learn that Hallie is dead. Codi is totally devastated, and she leaves town. Fortunately, fate intervenes, and her plane is sent back to Tucson after a failed engine makes everyone on board confront his or her own mortality. Codi catches a train back to Grace and flies into the arms of Loyd. Then she plans a kind of funeral for Hallie.
The novel ends with a flash-forward to a couple of years later. Codi is pregnant, presumably with Loyd's baby, and Doc is dead. Codi's caring for Doc's grave when Viola takes her to see the place where Codi's mother died, confirming a memory that Codi has had—and thought she invented—her whole life.