An experiment goes wrong and an unstoppable force is unleashed in the form of a giant orange iguana with fire breathing abilities and shape shifting powers. Rusty the Boy Robot to the rescue! Only the Japanese Boy Robot can't save the day - enter the American Big Guy! Action and fighting ensues, etc etc.
The cover might make the book seem like a kid's comic but I assure you it isn't. Geof Darrow's incredible artwork is extremely graphic, especially in the fight scenes. Frank Miller's script is alright but reads a lot like a pastiche of 50s/60s comics propaganda than a true representation of his own abilities (for Miller's best see Dark Knight Returns and the Sin City series).
More troubling is the insinuation of Japanese weakness and American superiority. Rusty is the best the Japanese can offer in terms of fighting and he is a comical figure, insecure, weak, and completely ineffectual. It takes the might of the Americans to come in and destroy the threat. The Big Guy is just that, a big robot that kicks ass and does what the Japanese boy robot can't. Big Guy's own reaction to Rusty is similar to that of Batman's to Robin in "All Star Batman" so it's a similarly interesting relationship where the Big Guy is clearly the hero. Either way I wasn't particularly fond of Miller's depiction of the ineptness of Japanese military power. It seemed unnecessarily jingoistic.
As for the book? A bit tedious in a way because it's the simple comic book hero story of hero fights monster, monster loses, blah blah blah. For a better collaboration between Miller and Darrow, check out "Hard Boiled", a much more interesting comic book. "Big Guy and Rusty" is, well, kind of rusty.