The story take place before Arkham Asylum and has little to do with Batman's story, but it is a little perplexing how he didn't notice Jim Gordon go missing for several hours. The Joker has taken Commisoner Gordon to the offshore funfair just outside of Gotham's Arkham asylum, along with 3 groups of officers.
This is where Batgirl comes to save her father's life. First you have to choose a random set of officers from three different groups, then Robin tells you there's a bomb. Off you go to disarm it and then it carries on until you've solved the final puzzle, having saved the final group. At this point you're sent to find Robin and Joker to get your dad back.
The combat is exactly the same as the main game with team ups between Batgirl and Robin, but the final boss fight seems, uninspired and dull considering you're up against the mastermind who turned himself in to a giant Joker-Bane hybrid. What is different, however, is Batgirl can use her hacking skills to turn almost everything in to a weapon to take down Joker thugs.
Returning are collectables from Asylum too, balloons, teeth and jack in the boxes have come back for you to hunt down and destroy, with no real reason than an excuse for an achievement.
What it does well is everything that Arkham Knight is good at, fast, free-flowing movement. Fluid control, but unlike the main game, the story is a little lacking and you really struggle to care about Gordon on several levels.