The book is a series of diary entries by each of the 4 main characters, Captain Zebadiah, programmer Deety and her math professor father, and an off-campus socialite. The odd foursome dash off in Zeb's sports car cum spaceship, outfitted with the professor's hyperspace drive, into various fictional universes. There is plenty of sex, rivalry, and even a trip to Oz.
The novel lies somewhere between parody and homage in its deliberate use of the style of the 1930's pulp novels. Many of the plot lines and characters are derived directly from the pulps, as directly referenced by the first line of the novel:
"He's a Mad Scientist and I'm his Beautiful Daughter." --Deety
Number of the Beast contains many in-jokes and references. For instance, the name of every villain is an anagram of a name or alias of Robert or Virginia Heinlein. The book is generally held to be Heinlein's worst novel, although one school of thought holds that it is an extended demonstration of Heinlein's knowledge of writing using bad examples.