The Fifth Musketeer

In this 1979 German-Austrian adaptation of Alexandre Dumas' final section, "The Vicomte of Bragelonne: Ten Years Later", the French King Louis XIV is unaware he has a twin brother, Philippe, and stages an assassination where his doppelganger is locked in the Bastille. When Philippe escapes, everyone believes him to be the real king, leading to numerous confusions and intrigues at court as Charles D'Artagnan and his musketeer friends try to free the mysterious prisoner and claim his rightful throne.