King Louis XIII is thrilled to have born to him a son – an heir to the throne. But when the queen delivers a twin, Cardinal Richelieu sees the second son as a potential for revolution, and has him shipped off to Spain to be raised in secret to ensure a peaceful future for France. Alas, keeping the secret means sending Constance, lover of D'Artagnan, off to a convent. D'Artagnan hears of this and rallies the Musketeers in a bid to rescue her.