Origins cover how the Templars came about. They do not mention Hugh de Payne or the original group who are credited with the founding of the Templars, but does pick up from the time of the First Crusades, and does cover the circumstances of why a military order was needed, and how the Order was finally acknowledged and accepted.
Corporation is much more interesting, showing how much of an important part the Templars played in the construction of the financial basis for that time period in history. The foundation of the Templar finances is examined and clearly laid out. This is necessary in order to understand what finally happens to them.
Frontier lays the basis for the fall of the Holy Land and the effects it had on the Templars. Reinforced and secure, they lose their ground to the Mamluks. The power struggle plays out, the Holy Land is finally lost to the Crusaders, and the Templars move back into Europe to find they are financially secure, but have no reason now for existence
The final installment, Legend, examines the fall of the Templars. Their financial holdings appealed to a failing monarchy, they become the target for the financially strapped French King, and are arrested, tortured, tried and burned.