Son of Henri IV and Marie de Médicis, Louis XIII ascends the thronevery young because his father was assassinated. The agreements of Fontainebleau very quickly oblige him to marry the daughter of the King of Spain, Anne of Austria, an arrangement which seals peace between two formerly enemys. He is only 23 years old when he calls the Cardinal of Richelieu to sit at the Council and to consolidate the royal authority contested by several conflicts through the provinces of the kingdom. For this reason he is not very worried by the administration of the country. If he wants sometimes to show that he is the King, he benefits also much from the pleasures of the Court.

The Three Musketeers: The year is 1625. The young D'Artagnan arrives in Paris at the tender age of 18, and almost immediately offends three musketeers, Porthos, Aramis, and Athos. Instead of dueling, the four are attacked by five of the Cardinal's guards, and the courage of the youth is made apparent during the battle. The four become fast friends and embark upon an adventure that takes them across both France and England in order to thwart the plans of the Cardinal Richelieu. Along the way, they encounter a beautiful young spy, named simply Milady, who will stop at nothing to disgrace Queen Anne of Austria before her husband, Louis XIII, and take her revenge upon the four friends. 
(Source: AniDB)

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Based on The Three Musketeers by Alexandre Dumas. 
Note: The pilot episode of Anime Sanjuushi.

D'Artagnan leaves his hometown of Gascogne for Paris in order to join the King's Musketeers or the Guards of the Cardinal. At his arrival, he went into an argument with Athos, Porthos and Aramis and provokes them into a duel. At the time of the duel, they were interrupted by the Guards of the Cardinal who are enforcing the King's decree banning duels. Afterwards, D'Artagnan and company became friends and adopted the motto, "all for one, and one for all". (Wikipedia)