If you need some zombies killed, who do you call? The Lucky Thirteen. A few weeks have passed since the events in Aurora, and Bree, Nate, and the gang are back on the road. The odd mall to raid, cattle to herd, provisions to deliver - all no match for our trusty group of scavengers. The world is yours for the taking when killing zombies becomes routine. Yet with temperatures rising in late spring, a new menace appears on the post-apocalyptic theater of war. Roving bands, thousands of zombies strong, are scouring the country, and already the first settlements are going dark. In a world where every human life is valuable, losing hundreds at once is not an option. What makes the zombies band together like that - and, more importantly, why are the tough ones even harder to kill than mere months ago? Bree may wish that she's just paranoid, but she can't shake the suspicion that finding a cure may not be the only cause the government is pouring resources into. But to what end?