It is the story of a Howard W. Campbell Jr an American Nazi propagandist and US counter spy during WW2. Howard is eventually held for trial in Israel and after the war lives in isolation in New York. Much of the discussion in the book centres on Howard having done his job as a radio broadcaster for the Nazi's a little too well.
The moral of this story, as pointed out by Vonnetgut himself in the introduction, is also the source of his most famous quote: "We are what we pretend to be, so we must be careful about what we pretend to be"
This is a compassionate novel (for Vonnegut) while retaining some of his normal satirical flavour.