The first novel featuring James Bolivar ("Slippery Jim") diGriz. A 'fix-up' and continuation of two short stories which originally appeared in Analog. DiGriz is a thief and conman who lives outside the rules of society, and who occasionally -- if reluctantly and in unorthodox manner -- acts as an agent for the Special Corps, a peace-keeping task force. His adventures (up until 1999) fill ten books, and these are probably the most popular of Harrison's books in terms of sales.
The Rat stories take place in some far future where the homeworld of Earth is an all-but forgotten legend, and where the Galactic League seems to be slowly disintegrating into a series of independent worlds with feudal societies - resembling the Empire of Asimov's Foundation stories. The technology of these planets consists of what Brian Aldiss calls "horrible Paleo-Industrial artefacts" (Trillion Year Spree): steam-driven wagons and robots abound in these societies which seem to exist in a far future Ruritania.
In this first novel, diGriz is caught during one of his crimes and recruited into the Special Corps. Boring, routine desk work during his probationary period results in his discovering that someone is building a battleship, thinly disguised as an industrial vessel. In the peaceful League no one has battleships any more, so the builder of this one would be unstoppable. DiGriz' hunt for the guilty becomes a personal battle between himself and the beautiful but deadly Angelina, who is planning a coup on one of the feudal worlds. DiGriz' dilemma is whether he will turn Angelina over to the Special Corps, or join with her, since he has fallen in love with her.