From Syuura:
During a field trip to the imperial capital, Tokyo, Kaga Nanao overhears the beautiful melody of a violin and immediately becomes captivated by it. In order to meet the mystery violinist, Nanao learns the violin and defies her father's wishes by attending the Marianne Music Academy for girls in Tokyo. Upon arriving at the school, Nanao comes across her mystery violinist who turns out to be Sagami Takenomaru, the violin prodigy and adopted son of the famous violinist, Sagami Chougo-sensei. However, Takenomaru isn't just a regular person, he's determined to complete a mission and fulfill a contract...

Note: "Naki Shoujo no Tame no Pavane" means "Pavane for a Dead Girl" and is a wordplay on the Japanese translation of a solo piano piece, written by French composer Maurice Ravel in 1899, called "Pavane pour une infante défunte" - Naki Ōjo no Tame no Pavane or Pavane for a Dead Princess.

Spoilers included summary:

Under a contract and as the messenger of the angels, Sagami Takenomaru is given the mission of gathering the "Tears of Maria". Several girls within this series have tears growing within their hearts therefore becoming victims in order to have their tear(s) gathered. Starting with his first victim, a pure and dreamy girl named Kaga Nanao, Takenomaru must continue searching for Maria(s) to fulfill his mission in spite of who the victim might be.

Girls in the Marianne Music Academy consist of a variety from girls that Sagami Takenomaru have no relation to and have not known to a childhood friend that acts as an older sister to him.