Under orders, the Musketeers defend Louis' traitorous brother Gaston after he slaughters a band of veteran soldiers in a Paris tavern.
The heartbroken owner Christophe demands justice - but the preoccupied King welcomes his formerly exiled brother home with open arms. As tensions ignite and secrets spill, Treville crosses a dangerous line and the Musketeers are forced to choose between their royal duty and fellow soldiers.
Meanwhile, Feron and Grimaud spy a kindred spirit in Gaston.