The tale chronicles the battle to take the Citadel from the rogues on Postale V From the perspective of 3 different groups:
The Space Marines of the 3rd Company, marching on the citadel, ignorant of their goals.
The Ravenwing of the 2nd Company, Hunting down what they know only as a traitor to be excorcised of his Daemons in order to redeem him in the eyes of the Empress and the Lioness
The Deathwing Terminators and a Venerable Dreadnought from the 1st Company, hunting down one of their fallen brethren to be interrogated by the Interrogator-Chaplains of the Reclusiam.