From the Introduction:
Twilight Crusade is an adventure in three parts that focuses on the actions of Vir Modren and the politicking between his vassals. It takes the Explorers on a Grand Endeavour into the heart of Modren's Realm, alongside the acolytes of an ardent Inquisitor and his unexpected allies - a hunter cadre of Tau - from the far-flung reaches of the galaxy. The Explorers must venture deep into Modren's domain and negotiate the power-struggles of his many followers in order to gain entry to his personal fortress, where they can confront him directly.
The three chapters of this adventure form a Grand Endeavour that provides the GM with a framework to gauge the progress and success of the Explorers as they venture into Modren's Realm and navigate the intrigues of Modren's vassals. This Endeavour can be modified to suit the GM's needs and broken down into smaller sections or padded out with new goals depending on the actions of the Explorers. A detailed breakdown of the Grand Endeavour's objectives can be found at the end of each chapter of the adventure.
In addition, this adventure gives GMs the perfect opportunity to incorporate the content of the Tau Character Guide, as the warriors of that xenos race are featured extensively throughout.