It's hard being the older step brother to a pair of demonic sisters. It's even harder having to defend them from the power hungry denizens of the demonic underworld. Such is the life of Basara Toujou; and he's done a pretty decent job of managing it so far.
Shinmai Maou no Testament Burst directly follows the events of the previous season, with Basara and his sisters Mio and Maria Naruse attempting to enjoy a somewhat normal life. The best laid plans of demons and men oft go awry however, and Basara, Mio, and their friends begin to face a number of new threats.
As if the situation couldn't get complicated enough, Maria's older sister Rukia shows up, insisting that Basara and the Naruse sisters must come to the realm of demons.