It's hard to fight an invisible enemyErin O'Reilly's squad has filled more than a few cells at Riker's Island with violent thugs, mobsters, and murderers. The New York prison is notorious for overcrowding, corruption, and violence. But Erin and her team haven't worried about the crooks they've locked up-until now.A dirty cop is viciously stabbed to death in the Riker's infirmary. It has to be an inside job; evidence points to an unholy conspiracy of inmates and crooked Corrections Officers. Erin isn't sorry the bad cop is dead, but the murder is only the first act in a desperate power struggle for control of the remnants of a once-powerful Mafia family.While Erin, her K-9 Rolf, and her fellow officers try to stem the flood of violence, they find new trouble in their own station: a tough Internal Affairs cop has come to clean house. The new Lieutenant is willing to do whatever it takes, even if it means tearing the Eightball right down to its foundations. With enemies on all sides, it will take all Erin's grit and determination to stop heroes and villains alike from shattering everything she's worked to build.