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".

(Source: Wikipedia)