In order to move to story forward, you will of course have to go to the Gotham City Police Dept. #1 which you have already visited previously in the game. As you might have guessed, new enemies have appeared in the area and as a result you will have to take care of them. I'd suggest attacking the inmates from above, aiming at the armoured one #2 and afterwards quickly finishing him off.
After the proper part of the fight begins, start attacking the normal enemies, but look out for the canisters #1 and other big items that they throw. In the end take care of the ones with shields #2. You need to use the same tactic as before, so stun them first (B) and afterwards crush to the ground (2xA).
After you're done with them, choose the Remote Electric Charge and use it to partially raise the gate #1. Start running towards it and slide underneath (right trigger). Turn right and choose the door leading inside the Gotham City Police Dept. #2.
Head to the main room of the building #1 and approach Mister Freeze to initiate a conversation. The meeting will soon turn into an argument, as Freeze won't want to give Batman the only cure until he doesn't save his wife. As a result, you will have to fight him.