Human Trials found the human race scurrying to Mars and facing horrible trials in the wake of extreme overpopulation. In its sequel, Human Expansion, humankind has not only survived but is also once again thriving, thanks in large part to a series of interconnected rings that allow instantaneous travel between worlds and systems. Dozens of planets are now inhabited, with the "Core" systems containing the most wealthy and influential populations.

The Exploration Service searches for remnants of the race that built the rings. When a trader named Jown discloses to them a deserted alien base inside a hollowed-out asteroid, they uncover clues that indicate a prehistoric enemy of the human race may not be as ancient as they previously thought.

Back on one of the "Core" system planets, Elizabeth "Ellie" Goodwater is an only child and heiress to the Goodwater estate. Her life couldn't be more different from Jown's impoverished and hardworking existence. But these two individuals, though separated by class and distance at birth, share a fate that is unexpectedly intertwined.

Experience humankind's next great trial, as it defends its existence against a powerful foe, through the vastly different lives of two unlikely heroes.