Kallus has always kept secrets form those who don't need to know about them so it just seems right that his son is never mentioned in any of his conversations. But, when Thrawn starts to becomes cirous as to how the rebels are learn more about secret empereal locations he starts to look more into agent Kallus's secret life out of the empire. Learning of this Kallus decides that its time to take Zeb up on his offer he made on the moon.
11 parts