When the planet Quaddravidia is attacked by the treacherous T’au, only one person can stop the xenos and save the planet for the sake of the Emperor: the legendary Hero of the Imperium, Commissar Ciaphas Cain. When the enemies ask for a temporary truce, Cain expects the worst—and his fears come true in the form of a horde of Tyranids. While their fleet moves closer to Quaddravidia, Cain must forge an alliance between the Imperium and the T’au—but can he trust these unfathomable xenos?