Dick Jarvis, the chemist of the historic Ares expedition to Mars, recounts a harrowing eight-hundred-mile solo trek across the red planet after his auxiliary rocket suffers a mechanical failure. Forced to walk back through barren deserts and ancient sea beds, Jarvis encounters an ecological system entirely divorced from Earth biology, populated by strange, crawling plant-animals and silent crystal spores.
Along his journey, Jarvis is rescued from a tentacled monster by a bizarre, bird-like Martian named Tweel. Despite mutually incomprehensible languages, the two form a deep and loyal bond. Tweel demonstrates a sophisticated intellect that surpasses human assumptions, particularly when navigating the hazards of the Martian landscape and recognizing alien life forms for what they truly are.
Throughout the march, Jarvis confronts two distinct forms of artificial and organic strangeness. First, he discovers ancient pyramid builders that turn out to be immortal silicon-based automatons consuming mineral matter and excreting stone bricks. Second, he falls victim to a shape-shifting dream-beast that uses the deepest longings and memories of its prey as a fatal lure, a trap from which Tweel barely manages to save him.
Their travels ultimately lead to a vast canal system controlled by barrel-shaped creatures that march in endless robotic routines and robotically repeat hollow phrases of friendship while exhibiting moments of bizarre hostility. Trapped briefly within the labyrinthine mound city of these barrel-people, Jarvis witnesses their ritualistic suicides and discovers a crystal artifact capable of healing diseased tissue with intense radiation.
Just as the harsh Martian night threatens to freeze Jarvis and the hostile natives close in with copper darts, engineer Karl Putz arrives in a rescue rocket. Jarvis barely manages to pull himself to safety while Tweel leaps away into the southern darkness. Though the expedition leaves Mars with lingering mysteries and profound scientific treasures, Jarvis is left with a quiet regret over losing his strange and loyal companion.
