Telepath Larry Greenberg makes a catastrophic error when he attempts to establish mind-to-mind contact with Kzanol, an ancient, malevolent alien entity. Kzanol, a thrint from a distant galaxy, has been imprisoned on Earth for two billion years and is now free. Larry, having inadvertently tapped into Kzanol's consciousness, becomes privy to the alien's terrifying plans to plunder and enslave Earth, the very planet Kzanol has targeted for his next conquest.

This novel is recognized as a significant entry in Larry Niven's 'Tales of Known Space' series, often lauded for its imaginative scope and the introduction of the formidable thrint species. Readers frequently highlight the chilling concept of an ancient, powerful alien with destructive intentions and the unique predicament of the protagonist who becomes intimately aware of the invader's mind. The narrative's exploration of advanced alien biology and societal structures, particularly the thrints' predatory nature, is a common point of discussion among fans. The story is generally appreciated for its classic science fiction themes and Niven's characteristic world-building.