In the 22nd century, the people of Earth inhabit giant, fusion-powered cities sprawling across the planet. In addition, they are hosting the Cephians: advanced plantlike aliens who have come to assess humankind's readiness for joining their galactic community. Although they are friendly, the aliens have been in the solar system for forty years, and humanity's patience and trust are wearing thin, at times violently so. Eminent astrobiologist Dr. Shana Savarino feels otherwise. She admires the Cephians' quiet, strategic wisdom. She also has environmental concerns for her planet, as well as long-standing and personal issues with humans, and so she prefers the company of the visitors. Meanwhile, human scientists discover a mysterious object buried deep beneath a giant crater in Mexico: a powerful, sentient artifact that harbors a secret concerning humanity's origins. Accidently awakened by nuclear bombs, the object now seeks to wipe out all life on Earth, and it seems only Shana has a chance of stopping the disaster.