They set off a series of earthquakes using a stolen seismic vibrator truck from an oil firm.

Judy Maddox, an FBI agent, is the only one who can stop them and the rest of the story revolves around how she tries to do so.

Carlos Ramet noted that the character of Judy Maddox, one of the novel's main protagonists, is an example of the a change in Follet's writing that begun with The Third Twin, i.e. "the change in emphasis from male to female perspective" and the development of capable and heroical female characters.

He also observes that Maddox's contempt for her superiors, described as the "suits" and "the big shot[s]" from Washington", is representative of Follet's "post-war British ambivalence towards American materialism".

With regards to the novel structure, Ramet argues that it combines the "formulaic hunter/hunted device" with "Buchanesque and Hitchcockian elements", and also shows inspiration from Ian Fleming's works.