Sixteen-year-old Rumi Sabzwari has spent his entire life behind the armored walls of St. Iago, which protect citizens of the Union of Upper Cities from the outside world's environmental devastation. But when rebels infect his father with a fatal virus, Rumi escapes St. Iago, desperate to find a cure.

In the ruined city of Paraíso, Rumi meets fifteen-year-old Paz, who agrees to guide him on his journey. As they travel together, Rumi finds himself drawn to Paz--and behind her tough exterior, she begins to feel the same way. But Paz knows more about Rumi's father's illness than she's saying and has her own agenda. With the powerful forces at play in their cities putting them at odds, can the two learn to trust in each other--enough to imagine a different world?