On a typical afternoon, teenage Joe is being chased by bullies from school when he accidentally runs through a portal to another world. He finds himself in a training camp for a group of teenage boys and girls who will become Runners.

Each cycle, three Runners are sent to place the power sources that will secure the shield that protects their land, Carcassia, and keeps the enemy Kabanians at bay. Once they've achieved their objective, a toxin is released into their bodies and they die. No one is permitted to return through the shield. And nobody can remain in the enemy territory.

Joe is sent on the next Run, accompanying the Runner, Kaia. He is hoping that he will find a way back to Earth - much to Kaia's annoyance, as she is focused only on her goal. Gradually, a delicate friendship develops between the two, and Joe wonders if he can save Kaia as well as himself. But as they pursue their Run, Kaia and Joe come to question the histories they've been given, and the ambition of their enemy. The Run suddenly becomes more dangerous than anyone could have envisaged.