In the Underground, old age is impossible and a quiet, painless death unheard of. Between pounding pistons, grinding gears, and scorching steam, not to mention the labyrinth of the Underground itself, one cannot hope to survive for long. Sparks is a Sootsprite, and worker in this hellish world below the City. She is a Breadwinner, a thief who serves as a main provider for her people. As one such leader in her society, she has become a favorite among both the younger and older workers. Little do they know, evil is stirring on the corrupted Surface. When a mysterious figure reappears in her life, she begins to understand what truly happened to her father, who vanished early in her childhood. Her life is further complicated by Cole, a Surfacer dignitary who has escaped his horrible family and accidentally slipped into the dangerous realm of the Underground. Everything they both know is a lie, and as their destinies intertwine, it will take everything they have to untangle themselves from the futures neither of them want and save their two very different worlds.