It's about CA trying to get out of the Wildstorm universe, because a) he wants to return home and b) something, somehow, turned him into a living bomb, and when he goes off, the entire Wildstorm world will be destroyed. As he goes around trying to get back, he meets up all the big-shots of the Wildstorm universe, including Majestic, the WildCATS and the Authority.
The writing is superb, from the dialogue to the characterization to the fights to the very cool ways he uses his powers to the interaction between characters. Although CA ends up fighting everyone he meets, the encounters are still made to be believable, and doesn't reek of the same fight-for-fight's-sake stench that plagues Supergirl. The book also explores the stranger-in-a-strange-land aspect, with Captain Atom shocked and horrified at the drastic difference in the super-hero community between the WU and the DCU, particularly at the fact that the human population in general is terrified of super-heroes. And then there's the fact that Captain Atom hasn't been this badass in decades. All in all, it's cool as Hell.
To top it all off, the art is superb.