From the back cover:
A hundred years ago, Primus Adrienne Sims of ComStar suffered nightmare visions of a dire threat to ComStar and the Successor States - hordes of strange monsters from beyond known space, invading the Inner Sphere to destroy it. These visions so troubled the Primus that she created the Explorer Corps, a division of ComStar dedicated to probing the Deep Periphery, to hunt down the threat she had foreseen.
A century later, the Clan invasion made the nightmare come true - and gave the Explorer Corps a vital role in the Inner Sphere's survival. With the backing of the Draconis Combine, the Explorer Corps has expanded its operations and embarked on a massive hunt for the Clan homeworlds. Small mercenary units from all over the Inner Sphere are joining the search, hired by the Explorer Corps for secret missions into the Deep Periphery - missions from which few return...
The Explorer Corps sourcebook describes the Explorer Corps' history and structure, including key personalities, operational procedures, and the Corps' efforts to locate the Clan homeworlds before the Truce of Tukayyid expires. Also included is background material on life aboard a 31st-century spacecraft and descriptions of various colonies and Clan bases in the Deep Periphery and other inhabitants of that little-known region of space. Special game rules for BattleTech, MechWarrior, and BattleSpace are designed to enhance campaigns involving space travel or planetary exploration; new material includes rules for creating new solar systems and planets to explore, as well as new types of vehicles and 'Mechs.