The era is Meiji.

Sana Shirafuji, who lost her grandmother and is left alone in the world,
knocks on the military's door to become a "Bride Thorn" to make a living.

A "Bride Thorn" is a "ration" for girls known as "Vampdolls",
who are specialized in combat.
Unaware of this, Sana accepts the military contract out of desperation.
On her first day of enlistment, she meets
Yukine Hanafusa, a beautiful yet disdainful Vampdoll.

Gradually, Sana and Yukine begin to understand each other.
On the first night of blood drinking,
they discover that the act of drinking blood brings physical and emotional exhilaration.
Despite their awkwardness, they experience their first blood drinking together.

Some time later, Yukine becomes incapacitated during a certain operation...

[From Steam]