After a decade of unbroken successes, the world-conquering armies of the Drouinistes, led by the former thief John LeClair, are finally being turned back.  The dwarves of Thelvenheim have lifted the siege of their great metropolis, while the forest dwellers of Mulvenna under the leadership of the Grand Druid have liberated western Soucy.

A party of adventurers, pursuing fragments of notes left by a Drouiniste cleric in Beaudin, learns of a terrible artifact, the Silver Moon. Created by the ancient and extinct people known as the Wanderers, the Silver Moon has the power to drown Teravide in a great flood.  Such a flood destroyed the Wanderers themselves millennia ago, as the party discovers after raiding the home of the witch Jeanne Rheaume and consulting an unnaturally venerable aquatic elf in his watery village.

The party finds itself in a race to find the Silver Moon and destroy it before it can fall into the hands of John LeClair.  Pursuing leads left by a murdered sage, they unearth the tomb of the Wanderer king Allum underneath Thelvenheim.  There they find more clues and a portal to the Elemental Plane of Air, where the Silver Moon has been guarded in the ruins of a cloud giant's castle.  They defeat LeClair's agents, acquire the Moon, and learn that it must be destroyed by exposing it to the light of the sun "undimmed by clouds or winds"; i.e., in the vacuum of space.

Returning to Allum's tomb, they use a teleportation circle to reach the abandoned but still intact domed Wanderer city of Tustin, atop a frozen peak on an uncharted island.  They master the Wanderer technology they find there, and activate a special gate which takes them to the greatest of the Wanderer temples: the moon itself.  Unfortunately, LeClair is there waiting for them.  The party is quickly captured, and LeClair prepares the ritual to activate the Silver Moon and flood the earth.

However, LeClair has missed a key component in the ritual, and inadvertently activates a stained-glass golem in the dome of the temple.  In the resulting three-way melee, LeClair's henchmen are killed, the golem is destroyed, and LeClair himself is polymorphed into a dove.  The party's half-orc thief exits the temple's airlock with the Silver Moon, destroying it and killing himself in the process.  The party returns to Thelvenheim with LeClair in tow, where the city is celebrating the defeat of the Drouinistes at the hands of General Per Samuelsson.