A group of astronauts travel to Osiris IV, a planet with an atmosphere believed to be similar to Earth's. Earth is dying out, so there are reconnaissance missions to other planets in order to determine whether they can be colonized.
When they arrive, the planet appears to be devoid of life...but then the astronauts discover the remnants of a civilization BELOW Osiris IV's surface: Millions of miles of inter-connected steel corridors that once housed a very scientifically-gifted race of beings and all their highly developed machinery and inventions.
The only life that remains in the corridors is a deadly mutation of the original alien species: quadripedal predators that are somewhere between lizards and tigers. The astronauts must fight these creatures, amidst their discovery of a way to use the beasts' blood samples to create clones of the aliens in their pre-mutated state...
Captain Bonner A. Betlar of the MWWV Elephant
Deceased Captain Thomas H. Talley of the MWWV Elephant (flashbacks only)
Leland Movish, Senior Secretary of the Interplanetary Federation for Colonial Research
The Luminads, a highly intelligent "alien" race who once inhabited Osiris IV. At first, the crew of the MWWV Elephant suspects that the Luminads are aliens, but eventually it is realized that the Luminads originated on Earth thousands of years earlier.
The Sauroks, predatory beasts which are the result of mutations in Luminad test subjects involved in a genetic research study