You start this mission just southwest of wonder tower. You'll see a dead body on the ground level. Examine the body and then examine the blood next to it. Follow the blood trail around the back of this building and talk to the prisoner there.
The second victim is just Southeast of the courthouse. Head to the group of buildings there and you'll notice an alley in the middle of them on the north side (visible on the map). Drop down into the Alley to find the victim. Examine the body and then the knife on the ground nearby.
The third victim is in the alley just south of the Ace Chemical building. He will be down in middle of the street. Examine the body and then the bleach by his foot. Follow the trail of bleach to the alley just West of the words "Park Row" on the map. Interrogate the guy there and you'll get a waypoint for just Northeast of the courthouse.
The place you're looking for is a little hard to find. Drop down into the alley that is marked. Look into the alley from Catwoman's apartment, drop down and take a left to the end of the alley. There will be a door that you can go through here. Go into the house and then on the table, you will find a personal journal. Listen to it. To get out of the trap, there is a panel over by the crib. Hack it to open the gate and head out. Mission complete.
