Jupiter's moon Io has an extremely hostile environment. There are hot lava streams, seas of boiling sulfur, and frequent volcanic eruptions straight from Dante's Inferno, in addition to constant radiation bombardment and a surface temperature hovering at minus 180 degrees Celsius.Is it really home to a great danger that threatens all of humanity? That's what a surprise message from the life form discovered on Enceladus seems to indicate.The crew of ILSE, the International Life Search Expedition, finally on their longed-for return to Earth, reluctantly chooses to accept a diversion to Io, only to discover that an enemy from within is about to destroy all their hopes of ever going home.