Orion is a Subpar, expected to mine the tunnels of Outpost Five, near the deadly flash curtain. For generations, her people have chased cirium-the only element that can shield humanity from the curtain's radioactive particles. She and her caving partner Dram work the most treacherous tunnel, fighting past flash bats and tunnel gulls, in hopes of mining enough cirium to earn their way into the protected city.

But when newcomers arrive at Outpost Five, Orion uncovers disturbing revelations that make her question everything she thought she knew about life on both sides of the cirium shield. As conditions at the outpost grow increasingly dangerous, it's up to Orion to forge a way past the flashfall, beyond all boundaries, beyond the world as she knows it.