William Lee is an exterminator who finds that his wife Joan is stealing his insecticide (pyrethrum) to use as a drug. When Lee is arrested by the police, he begins hallucinating because of "bug powder" exposure. He believes he is a secret agent with two handlers or controllers in the forms of a talking insectoid typewriter and an alien "Mugwump". The bug assigns him the mission of killing Joan. She is allegedly an agent of an organization called Interzone Incorporated. Lee dismisses the bug and its instructions and kills it. He returns home to find Joan having sex with Hank, one of his writer friends. Shortly afterwards, he accidentally kills her while attempting to shoot a drinking glass off her head in imitation of William Tell.
Having inadvertently accomplished his "mission", Lee flees to Interzone. He spends his time writing reports for his imaginary handler, and it is these documents which, at the insistence of his literary colleagues, eventually become the titular book. Whilst Lee is under the influence of assorted mind-altering substances, his replacement typewriter, a Clark Nova, becomes a talking insect which tells him to find Dr. Benway by seducing Joan Frost. Joan, curiously, is a doppelgÃ¤nger of his dead wife.
After coming to the conclusion that Dr. Benway is, in fact, the secret mastermind of a narcotics operation for a drug called "black meat" which is supposedly derived from the guts of giant centipedes, Lee completes his report and flees Interzone to Annexia with Joan Frost.
Stopped by the Annexian border patrol and instructed to prove that he is a writer as he claims, Lee produces a pen. As this is insufficient proof for passage he inexplicably offers a demonstration of his William Tell routine using a glass atop Joan Frost's head. He again misses badly and thus re-enacts the earlier killing of his wife. The border guard cheerfully bids him welcome to Annexia. Lee is shown shedding a tear at this bittersweet accomplishment.
Who is William told he must kill?
Joan