In the Warring States period, Hotaru is a kunoichi from Iga sent to infiltrate Azuchi Castle — the seat of Oda Nobunaga. She ends up working directly for Nobunaga and maintaining a cover as the sister of his confidant Akechi Mitsuhide in order to stay inside. The castle and its inhabitants are not what she expected. Neither are the feelings she finds herself developing for the people around her.