The show mainly follows veteran firefighter Tommy Gavin (Denis Leary) and his ever troublesome family as they deal with real life issues, either with post 9/11 trauma or domestic problems. Tommy has to struggle with the loss of his cousin and best friend Jimmy Keefe, who died on September 11, 2001, and frequently visits Tommy in visions. In the pilot episode, Tommy and his wife have already separated and Tommy has moved across the street. He is ill-tempered, self destructive, hypocritical, manipulative, and a relapsed alcoholic.

Source: Wikipedia.org