Never leave someone behind: it's a promise easier made than kept, especially when seventeen-year-old Pip takes the headstrong twelve-year-old Iris under her protection in the wake of an earth-shattering plague. After an unspeakable tragedy, the duo must negotiate the complexities of their own identities amid the nearly unrecognizable remains of Spokane, Washington. When they're captured by a violent gang, Pip and Iris meet Fly, a stubborn and courageous older girl. When their captors exchange them for supplies at Thistle Hill Orchard, an idyllic farm turned commune, it seems that the girls' luck has finally changed for the better. But the proselytizing of Veronica, Thistle Hill's leader, and the looming presence of her right-hand man, Granville--who is more snake than cowboy--make the trio's circumstances more perilous.

As Pip, Iris, and Fly weigh the precariousness of their lives at Thistle Hill against the uncertainty of life on the outside, they simultaneously grapple with the secrets that make their situation all the more tenuous. Pip's vow to never leave someone behind may have made survival more difficult for her, but this promise could also be the key to finding meaning in the ashes of what came before.