From the publisher:

In the world of Mouse Guard, mice struggle to live safely and prosper amongst harsh conditions and a host of predators. Thus the Mouse Guard was formed: more than just soldiers that fight off intruders, they are guides for common mice looking to journey without confrontation from one hidden village to another. The Guard patrol borders, find safeways and paths through dangerous territories and treacherous terrain, watch weather patterns, and keep the mouse territories free of predatory infestation. They do so with fearless dedication so that they might not just exist, but truly live. Saxon, Kenzie and Lieam, three such Guardsmice, are dispatched to find a missing merchant mouse that never arrived at his destination. Their search for the missing mouse reveals much more than they expect, as they stumble across a traitor in the Guard's own ranks.