Issei and Rias' group prepare for their Ranking Game against Diodora but find they've been caught in an insurgent plot against the Underworld.
Heh.
Whenever a whole bunch of action is about to happen in this show, you have to pay attention to the dialog again. That's kind of difficult with all the fanservice thrown in your face, but most of the exposition happens when two characters are walking somewhere or standing around together. Case in point - Issei and Koneko coming back from a devil assignment.
Even in the middle of a great war with many factions, the devils still need to do their jobs when they get summoned. Because Issei was such a weak devil when he was first made, he rode around on his bike to his jobs, but now that he's much more powerful, his regular clients still expect him to show up on his bike as added "service." It was a nice touch to ground the show back in its universe and to keep reminding us of the events of the first season, in the same way Issei's frequent flashbacks of the devil who killed him keep doing.
Hey, that Diodora guy seems kinda shady. So says Chaos Brigade.