The sequel the The Three Musketeers begins with D'Artagnan still a lieutenant after 20 years of service. His seemingly promising career did not advance as he had hoped and he hasn't seen Athos, Porthos or Aramis for a long time.
The other three musketeers have however resumed their original names and effectively retired from service to civilian life.
After D'Artagnan is commissioned to serve as the military escort for the First Minister to the Queen of Austria, he sets out to recruit his former comrades back to military life. This sets the scene for the events that take place in the book.