A year has passed since the events of Arkham City, and Gotham has begun to show signs of recovery. The death of the Joker ushered in a period of relative stability for Gotham – there were even signs of regeneration, perhaps redemption. In truth the villains haven't disappeared, but without the Joker's culture of chaos it's been more difficult for them to destroy the lives of Gotham's honest citizens.
This isn't the salvation Batman has been fighting towards, though. It's a moment of respite. The Joker's absence created a power vacuum in Gotham, and the Scarecrow – arguably Rocksteady's most original take on a Batman villain – has returned to seize control. He's threatening Gotham with a large-scale chemical attack, forcing the authorities to issue a city-wide evacuation. This is how Arkham Knight begins.