It is also a gentle and humorous introduction to combinatory logic and the associated metamathematics, built on an elaborate ornithological metaphor.

Smullyan's exposition takes the form of an imaginary account of two men going into a forest and discussing the unusual "birds" (combinators) they find there (bird watching was a hobby of one of the founders of combinatory logic, Haskell Curry, and another founder Moses Schönfinkel's name means beautiful spark (or possibly beautiful little-finch).

Each species of bird in Smullyan's forest stands for a particular kind of combinator appearing in the conventional treatment of combinatory logic.

Deep inside the forest dwells the Mockingbird, which imitates other birds hearing themselves.

While the book starts off with simple riddles, it eventually shifts to a tale of Inspector Craig of Scotland Yard, who appears in Smullyan's other books; traveling from forest to forest, learning from different professors about all the different kinds of birds.