As the Fifth Frontier War locks the Spinward Marches in mortal combat, a band of adventurers journeys outside the Imperium on a mission of vital importance. Their destination: the planet Eshaar: a dangerous and hostile world where temperatures average 150°C. where sulfuric acid takes the place of water, and where enigmatic aliens hold the key to success or failure for the Imperium.

The mission: discover — and secure control of — affas, a mineral compound that could turn the tide in the Fifth Frontier War.

The opposition: the devious Zhodani, who will stop at nothing to ruin the Imperial cause.