Thrown on her own resources by the loss of her family, Ellen decides to hitchhike to her Aunt Sarah in Kentucky.

At a black castle in the forest, Ellen encounters a medieval device called the Hieronymus Machine which is mind-controlling people and turning them into arsonists and assassins for an unknown purpose.

In the original version, one of the adult characters speculates that the Machine was responsible for sending the crown to Ellen and arranged events to draw her to the castle.

O'Brien later revised the ending of the novel for the British edition to eliminate this section; Ellen's Aunt Sarah simply says she found the crown at "a little curio shop in Barcelona", with no apparent link to the Hieronymus Machine.

Kirkus Reviews observed "That Mr. Arthur's nightmare of thought control requires eight pages of explanation at the end is symptomatic of a fundamental failure ... [M]uch of what happens to her [Ellen] seems, even in retrospect, unnecessarily outlandish, and the connection with actuality is crude".