After getting to the steel mill and trying to go in through the tall chimney which you used last time, you realize you cant go in through there since the heat the furnace is generating is too powerful. You will have to find another way in. Contact oracle and ask her to find you a route into the steel mill. She will tell you that the only other way in is via the heat exchanges on the south side of the building. The location will be added to your map.

Go to southern side of the steel mill. There is a door there. However it is heavily guarded by armed thugs and by snipers above it. You have to take care of the snipers first, then defeat the goons on the ground. Do NOT attempt to  fight on the ground until you have taken out all the snipers. After defeating both the snipers and the thugs on the ground, enter through the door.

Once you enter the steel mill, there will be some enemies inside. Target enemies with the freeze blast to paralyze them momentarily. Follow the onscreen prompts to quickfire the freeze blast.

You will now reach a river of steaming water. Throw a freeze grenade into the water, then hop onto the ice block. The current will carry you the right direction, but if you want to move faster then fire your batclaw on the ring to pull yourself into that direction. duck when you approach the barrier. Shoot another batclaw to pull Batman towards the bridge, then jump onto the bridge.

Next, create another block of ice using the freeze grenade. There is a big drill in the middle of the body of water and batman must be careful not to touch it. Use the rings on the wall to maneuver batman around the drill, and climb onto the platform to the north.

You will hear inmates talking about how Joker looks healthy now. The cure must have worked for him! Also, oracle will be sending you updates about Protocol Ten.

Observe the window, behind it there are some goon. Glide and break through the window with a glide attack. Defeat the thugs and see a small tv broadcast by Joker who is looking much better.

Advance into the next room, where two of joker's guys are holding a prisoner of the opposing faction. Defeat them, and use the controls to lower the prisoner they were interrogating and save him.

Find the destroyable wall and use explosive gel to blast it. There will be another body of water you must now traverse. Create an ice block again. Use the rings to navigate along the river, avoiding the drill and ducking beneath the barrier. Work your way towards the ledge.

You will hear an update from Oracle, as well as an announcement by Strange that protocol 10 will begin in 30 minutes. You must hurry!

Use your line launcher to cross over to the other ledge. Use Freeze Blast to disable the steam pipes that are blocking your way. Disable the next steam vent as well using the freeze blast. Use an ice raft again to get to the next ledge.  There is a fuse box behind a fence. A remote control Batarang can hit switches that are out of reach. Throw the remote controlled batarang, maneuver it through the electric current to have it become charged with electricity, then drive it into the fuse box to disable it and open the gate. Use another ice raft to maneuver batman through the water, then grapnel yourself to the corridor on the right side.

Hack the door using the cryptographic sequencer and cross to the other side of the steel platform. You will run into Dr. Stacy Baker, the hostage you rescued earlier. She will warn you that the enemies have been laying a trap for you, placing mines on the floor.

Go through the door and enter a big room filled with armed thugs. Observe some of them are armed with proximity mines. Proximity mines are explosive charges connected to motion sensors. Any nearby movement will cause them to explode.

Try to ambush each of the thugs separately from the catwalks using drop attacks or inverted takedowns. When possible, hide with corner covers and perform silent takedowns from behind them. If you have the mine detonator the undercover cops gave you, use it to detonate mines for distraction.

After defeating them, make your way to the top of the room and open the doors there using your REC electronic gun. Open the next door too, then grapnel yourself to the platform and crawl through the ductworks. You will reach another big room.

Enter the box and travel on the conveyor belt. Locate Harley Quinn on the top of the platform and ungag here. She will reveal that  Joker took stuff from Mister Freeze and that it is in the Boiler Room. This will unock a new side mission, Hot and Cold in which you have to locate the Freeze technology stolen by Joker in the steel mill.

Jump down the shaft and go through some corridors and doors, making your way into the loading bay (bring up the map if you get lost). There will be several of Joker's thugs in this room. Observe the two red lasers coming from above signifying the snipers. You can either sneak up into the room from beneath and surprise them with a double takedown, or fire your REC electric gun on the electromagnet to disarm them. Then deal with the three goons on the ground.

Grapnel up to Joker's office and go through the door. You will meet Joker. There will be a cutscene followed by a fight. Joker will be easy to beat, but suddenly some trains will enter and many of Joker's thugs will join the fight, including a TITAN thug, Abramovici, and  Mister Hammer. Dont aim any attacks on Joker for now unless he is attacking you. We recommend first taking off the other goons and then focusing all of Batman's attack on Mister Hammer, and then do the Titan Thug. 

Listen for the signal bells. When they chime, it means a train is about to come through. Make sure to avoid the tracks when you hear the chimes as to not get hit by the train. You can also try luring some thugs onto the tracks so that they get hit.

Once you have defeated all the thugs including Mister Hammer and the Titan Thug, turn your attention to Joker and finish him off. There will be another cutscene. Joker will cause the ceiling to collapse and batman will be trapped underneath the debris. Joker will approach batman with a knife, intending to kill him, but suddenly Talia will come in proposing Joker to take her instead. She suggests a trade. Joker will free Batman, and she will give him the secret of immortality in return. Batman objects, as this will mean the Joker will be unstoppable.