Batman: Arkham Asylum Guide - Walkthrough
The Flooded Corridor
As you enter this area you get the Riddle: “The Rat Catcher needed more than just his charm to lead his army.”
As you enter you get a brief CS with The Joker in which you warn him, and he exits, setting off an explosive charge that blocks the corridor he took. If you switch on Detective Mode while you are crossing the room you will note the glowing duct grate to the left of the debris – but be sure you kill the two Dancing Teeth before you you rip the grate off the wall and exit the room.
Moving through the duct you will notice at the right turn a section of the floor that is barred grate – looking down into it you should see a gas mask and a pair of rubber gloves – scan these to solve the Riddle for this area!
As you approach the end of the duct it splits – take the right turn and grab the Riddler Trophy and then double back and continue on, and kick the grate out, dropping down into the Ventilation system room. You should spot the weakened wall ahead – gel it and blow it to pass through, and then take the door here to the Abandoned Chamber.