At the end of the previous part of the walkthrough, Batman was tricked by Joker in the Steel Mill and knocked unconscious by Harley Quinn.
Around this stage, if you have the Catwoman DLC, the game might switch to a Catwoman episode. We will cover the walkthrough for Catwoman separately. For now, we will focus on the Batman Story.
Batman wakes up and there is a cutscene with Joker and Harley Quinn. Joker has injected Batman with some of his blood to "motivate" him to find a cure. Batman doesnt care that they both die, however Joker has sent samples of his blood to emergency rooms all over the city to take Gotham down with them. Joker pushes batman out of the building. He tells Batman that Mister Freeze has the cure.
Batman messages Alfred telling him Joker has poisoned him and may have poisoned Gotham as well and that Mister Freeze has the only cure. Batman has calibrated the cowl to track heat signatures and find the coldest point in Arkham City
The batcomputer will now scan for coldest point in Arkham City. The Climate Analyzer will show up. Your new primary objective will be:
Locate Mister Freeze and recover the Cure
Track Freeze's location by identifying the coldest point in Arkham City.
Move in different directions and see how the climate analyzer reacts. When it gets colder the numbers will be bluish/green. If its getting warmer the numbers will be red. Keep heading in the direction of the coolest point.
When the temperature in your climate analyzer reaches around -4.500 to -5.000 then you will have found Mister Freeze's location. Turns out he is in the old GCPD building. He must be using one of the old forensic labs. There will be lots of thugs in the area and they will be armed. Approach cautiously. Try to get at them one at a time and then disappearing. Note that enemies will check in grates if they find a nearby unconscious body. If detected, grapple up and swing quickly between vantage points to escape. It is important that you dont let the armed men shoot at you through the grates as they can kill you fast with their automatic guns. If you do die, it will say "Mission failed" and ask you if you want to retry or quit to the main menu, so just hit retry.
Once the coast is clear, make your way to the south of the building and locate a garage door that says GCPD Vehicle Access. Note the mechanic motor above it. Shoot at the motor using your REC electrical gun just as you did in the Steel Mill.
When you go in you will hear Penguin's voice on the radio of one of the goons. Take the Encryption Card from the Penguin Radio. If you hack that radio, you'll be able to listen to all of Penguin's communications. Use the cryptographic sequencer to tune and decode the broadcast. This will hack into Penguin's Puffin-Zero channel.