“Every Villian Is The Hero To His Own Story”
In Injustice: God Among Us Superman after losing Lois Lane is driven into madness and becomes a Villian for humanity he loses his sense of justice and forms a Regime in which he unified the world government to eliminate all kind of crime. and declare himself the high councilor.
Batman tries to confront superman about his act being similar to a dictator but all goes in vain. That’s when Batman creates an insurgency of superheroes against Superman Regime. The insurgent ends the Regime rules and caged superman in a prison emitting red sun radiation to suppress his powers
Though a lot has happened in Injustice 1 if you still want to know more about the story of the first game you can simply play it or if you are just too busy to play you can read it here in Wikipedia section it explains it quite well.
Injustice 2 is set five years after the defeat of Superman. where Batman and his team are still fighting against the members of the regime who are trying to restore supermans rule. amidst this chaos, new villains have appeared and have collaborated with the remaining member of Regime and are now trying to overtake the world. and with the appearance of Brainiac and Darkseid, the troubles just seem to be rolling in for the insurgency.
Injustice 2 shattered alliance part 1:
The first trailer shows the ongoing conflict Batman and Superman. and also showcase the dilemma of supergirl whether to help his kin in his injustice or fight against him.
Injustice 2 shattered alliance part 2:
The second part show the arrival the formation of insurgency its shows arrival of Green lantern, Green arrow and also shows a new character Dr. Fate