At the dawn of the empire, the Emperor feared for the future of the country that he had created and so he used all of the resources at his disposal to create 48 tools and weapons of incredible power to protect it. So great was the power of these items called "Imperial Arms" that an iron rule emerged - when two of their wielders faced one another with intent to kill, one of them would die in the ensuing battle no matter what.
The iron rule of inevitable death is in full force as Night Raid, each wielding an Imperial Arm of their own, barring the Boss and Tatsumi, hunt an Arm-using serial killer who plagues the capital. Things turn even deadlier when Sheele and Mine are ambushed by an Imperial guard with an Imperial Arm in the form of a monstrous dog.
Along the way, more information about Leone and Akame's histories become apparent.
Tatsumi is sent to another series in the bonus chapter, in this case Dusk Maiden of Amnesia