This story is a one about a young girl named Sushi Portorate and a young boy named Yurito Gushita. 

Sushi is a Russian girl. Her family was often hired to kill people or steal, but because of this they were ordered to be killed by the Russian government. She was the heir to her family, and the only one who survived.

Yurito was a Japanese boy. His family was often hired for spying or information gathering, hiding was their specialty; the reason why they were never found by the Japanese government. The government that greatly wants them dead.

These two families work together. When the Portorate family was massacred, the Gushita wanted to find a memento. 

They found Sushi barely alive. She was traumatized and was left silent beyond natural, but perhaps her life will become more brighter within Yurito's presence.

If you wish to read you would be able to find out what would become of these two:

Yurito the heir to the Gushita family, intellectual and soon to be famous spy slash assassin.

Sushi the heiress to the Portorate family, silent and soon to be famous assassin slash mercenary.