It's an anime series and tells the story of two female assassins Mireille Bouquet and Yumura Kirika who call themselves Noir. Kirika is only maybe 15-16 years old and she doesn't even know her real name or age; she just woke one morning in a apartment without any other memories than that she is called Noir. She also found a japanese student ID card with her picture and the name Yumura Kirika in it. Despite her age, she also is a highly trained killer - even better assassin than Mireille.
Mireille is a daughter of the leader of a Corsican crime syndicate. When her family was killed and the syndicate destroyed her uncle took her to Paris and trained her in his profession - killing people. She took the name Noir that already was a legend in the underworld but seemingly wasn't used at the time by anyone.
The story begins when Kirika contacts Mireille and asks her help in finding out who she really is. Mireille's interest is captured by a pocket watch Kirika has; she obviously recognises it but I don't start spoiling why. Throughout the series they have to find the answers to questions like:
Who is Kirika?
Who are 'Soldats'?
What is the origin of the name Noir?
What is 'Le Grand Retour'?
Does the murder of the Bouquet family have anything to do with all this?
Despite the amount of violence in the story (the main characters are professional killers, after all) the pace is very calm most of the time. Unlike in some other anime shows I've seen in Noir people don't exaggerate their emotions and also the dialogue is usually slow-paced. This together with an excellent sound track gives a very nice, thoughtful atmosphere to the whole series.