As surmised by the title, The Princess is a little girl who engages in fairy tale-like adventures with her teddy bear Mr. Whiffle. These adventures range from basic life in her marzipan castle to waging war with Mr. Whiffle against her other stuffed animals. She is shown to indeed have an adventurous spirit, as seen in various illustrations of the text. However, she is also shown as being rather emaciated, with hints of being hungry. These serve as bits to foreshadow her true self.
However, her life is soon threatened by a bogeyman-like presence referred to as "The Thing Under the Bed". This sets up the story for its three endings, known as the Good Ending, Bad Ending, and (rather fittingly) "The One with the Teeth".
As aforementioned, The Adventures of The Princess and Mr. Whiffle contains three endings that are meant to be read in linear order. They go as follows.
In the good ending of the narrative, The Thing Under the Bed appears over the Princess as if to violently murder her. However, The Thing Under the Bed only tickles her, thus providing a happy (if not disgustingly saccharine) ending. Until the reader moves onto the Bad Ending.
Prior to the Bad Ending, The Princess finds and cares for a kitten that disappears. The Thing Under the Bed appears and holds out its hand over her face. Something drips into the hand over The Princess' face, as if to imply that The Thing Under the Bed has murdered her kitten. However, the truth is revealed in "The One With the Teeth".
"The One With the Teeth"
The dripping is revealed to be nothing more than a handful of melting marzipan with which The Thing Under the Bed hopes to share with The Princess. The Thing Under the Bed also reveals that it has cared for the kitten, ensuring its safety. It desires to be The Princess' friend. The Princess, however, reveals her true character and graphically devours both The Thing Under the Bed and the kitten alive. Following this horrific event, she uses their bones to create a tent under which she and Mr. Whiffle have a tea party.