Momiji Fujimiya believes she's an ordinary high school girl, growing up in rural Japan until she's told she's really the Princess Kushinada and that her blood is mankind's most powerful weapon against the shape-shifting monsters, the Aragami. The centuries-old war between humanity and the Aragami is heating up again, so Momiji goes to Tokyo to join the government agency fighting the monsters. Complicating matters is the presence of the roguish Mamoru Kusanagi: as a child he was kidnapped by a powerful Aragami and given seven souls (symbolized by the 'Blue Seeds'). Their power enables him to protect Kushinada, as spilling her blood would imprison the Aragami for centuries, possibly forever. In battle, Kusanagi transforms into a werewolfish version of himself with sword blades in his forearms. In his more normal state, he shares a fractious romance with Momiji.