"Incarceron" refers to a sentient prison that seemingly randomly punishes or ignores the hundreds of thousands of inmates contained in its vast walls. Nobody escapes (though many have tried to make their way Outside), and life is constant war -- nasty, brutish, and short. Both the prison and its inmates fight over scrap bits of technology to make their lives easier (or in the case of the prison, to make new prisoners out of the dead). Finn can't remember anything from his childhood except a few visions of a different world glimpsed during strange seizures, but is convinced he was born Outside the prison.

Claudia is the daughter of the Prison's Warden, and lives in a world seemingly stuck in the late Middle Ages. In reality, it is the remnant of a world destroyed by catastrophe, whose inhabitants long ago decided to "halt science and progress" to avoid further devastation. No one but the Warden knows where Incarceron is or what it is like inside. Claudia's attempts to escape an arranged marriage to the Prince lead her to a conspiracy and convince her that her original fiancé (the true heir to the throne) was placed in the prison rather than dying in an accident as everyone believes. Her attempts to get him out of Incarceron, and Finn's to escape, put the two on a collision course that will change the lives of everyone inside and outside of the prison.

Aside from science fiction and fantasy, Fisher adds the breakneck pace and frequent cliffhangers of the thriller genre, as well as elements of intrigue. Meticulous attention to world-building will satisfy fantasy readers, while a myriad of robotic creatures and cyborgs will thrill science fiction enthusiasts. Both Finn and Claudia change as a result of their experiences throughout the book, and secondary characters are suitably complex to make them interesting. Fans of China Mieville's Un Lun Dun and Railsea will enjoy the combination of magic and technology, while fans of Bacigalupi's Ship Breaker will find Incarceron's post-apocalyptic rat race of scavengers pleasantly familiar. Even fans of period fantasies such as Libba Bray's Gemma Doyle books will find much to like in the mysterious magic of Claudia's world. Adults fresh off of reader the Hunger Games books should also give Incarceron a try.