When retired cop Hiraga Raizou gets a lucrative gig working for a shadowy government organization known as the SS Agency, he is in for a rude awakening. The first day on his new job, he comes across a grisly murder and a lone little girl who's far from innocent. She's Suzukaze Miharu, an 11-year-old master assassin - and his new partner! The arrangement is simple: she kills, he cleans up and hides any evidence. Sure, the pay is great, but how much blood is Raizou willing to get on his hands?