Saya's amnesia isn't a plot device, it's the real story, and the show knows this will destroy you when the truth arrives. Every episode peels back who she actually is, what she's actually done, and what the Red Shield actually wants from her, and none of it lands the way you expected it to in episode one. The monsters are secondary to watching a girl spend fifty episodes learning she's the thing she's been hunting, and by then you've loved her too much to look away.
Saya wakes up with a year-sized hole in her memory, unsure who she really is or what she's lost. When a mysterious stranger hands her a weapon and vanishes, her carefully constructed normal life shatters. The world, she discovers, is infested with Chiropterans: ancient shapeshifters that wear human skin and hunt our kind for blood. They're everywhere, hidden in plain sight, and no one else seems equipped to stop them. An underground organization called Red Shield has been fighting this war in shadows for decades, and now they want Saya on their side. But every answer about her past only raises harder questions. Why can she fight these monsters when ordinary humans can't? What happened during that missing year? And most importantly, which side is she actually on? Blood+ balances visceral action sequences with genuine mystery, shifting between horror and intimate character drama. The show moves methodically, building toward reveals that reshape everything you thought you understood about Saya's origins. It rewards viewers who pay attention to the mythology being constructed around her.