Follow the girl to the door at the top of the stairs. Once the door has been opened, step through the door and enter the Olympus Heights Sewers. There is a Gate Crank at the end of the sewers next to a corpse. Pick up the Gate Crank and attach it to the Gate Crank Socket on the wall by the closed gate. Hold the A button to turn the wheel, opening the gate.
At the end of the street, step around the corner on the right. The first Big Daddy of this level can be found in this area. The Big Daddies in this level are Elite Rosie, they may give you a harder time, since your health is constantly being lowered by Fontaine's saying of "Code Yellow." There are a couple of turrets in this area that you can hack and use to your advantage, though they won't stand much of a chance against the Rosie's Rivet Gun. At the very least, they would serve as an adequate distraction. If you have been rescuing all Little Sisters encountered thus far, after saving this one you will receive another gift from Tenenbaum.
Nitro Splicers make their return in this level. Great, and unfortunately, the old Telekinesis move isn't as effective against them. A Gatherer's Garden machine can be found at the back of this area. Enter the Bistro Square across from the machine. There is an audio diary (Mozart of Genetics) on the counter in front of you as soon as you enter. Exit the Bistro through the door opposite the one you used to enter. There is a Circus of Values machine here that can be used to purchase some supplies if you are in need.
To Suchong's Apartment
Once you are ready, head down the road leading to Mercury Suits. There is a U-Invent station on the way, which you can use to create some ammunition, if you have any materials. At the end of the road, you are forced to face a large group of Splicers. Once they are dead, enter Mercury Suites. As soon as you enter, destroy the Security Camera on your left. A second Big Daddy occupies the proceeding area, so you can rescue or harvest the level's last Little Sister here. A Security Camera hangs on the large flight of stairs/elevator in the middle of this area. A "Power To The People" Weapon Upgrade station can be found on the side of the back side of the elevator.
Power to the People
Now, if you did not kill Sander Cohen (if you were following this walkthrough, you shouldn't have) at the end of the Fort Frolic level, you can earn two achievements (Irony and Found Cohen's Room) and use another Weapon Upgrade station. Unfortunately, if you killed Cohen, you cannot earn these two achievements, nor can you utilize the upgrade station, effectively robbing you of the Weapon Specialist achievement as well.
The door to Sander Cohen's apartment is along the wall to the right of the elevator door. Disturb the two dancers to lure Cohen downstairs. Kill him and take a picture of his corpse for the Irony achievement. On his body, you will find some ammunition and Sander Cohen's Muse Key. If you wish, return to Fort Frolic and use the key on Sander Cohen's Muse in the Atrium. Inside, you'll find some ammunition and money. Really, nothing you couldn't find anywhere else, so not worth the trek back to Fort Frolic. Use the Weapon Upgrade station on the wall far left of the bed, then exit Cohen's apartment.
Enter Culpepper's apartment, the door being directly across from the Weapon Upgrade station on the back side of the elevator. Move through the foyer and turn right, pick up the audio diary (Artist Woman) on the coffee table. Find Suchong's apartment and step inside. Upon entering, turn left and enter the room here. Hop over the rubble and pick up the audio diary (Fontaine's Human Jukebox) on the ground. Head back to the foyer, step into the flooded area and move to the back. Take the Clever Inventor tonic and the audio diary (Mind Control Antidote) that are both on the desk.
Audio Diary and Tonic
Finding the Remedy
Take the stairs up to the very top and move towards the U-Invent machine. There is an audio diary (Fontaine's Breakup) on a box near the U-Invent machine. By listening to this audio diary, you discover that the keypad code to Fontaine's apartment is 5744. Head back down to the first floor and enter the code on the keypad. When the door opens, take the elevator up to the apartment. Clear the room of any Splicers, destroy the camera above the door and proceed into the next area. Turn right and enter the room there, take the audio diary (Sad Saps) that's on the coffee table. Head up the stairs in the foyer and go through the door on the left.
Enter Fontaine's office and take the Lot 192 formula on the table, and the Electrical Flesh 2 tonic on his desk. Taking the formula returns your health bar to its normal size, but in turn your plasmids become unstable. You are no longer able to choose which one you have equipped. You will receive a random plasmid (even one that you may not actually have) and are unable to change to another plasmid. If you have been using plasmids as your primary weapon throughout the game, this may be a problem.
All in all, only slightly irritating, since even if you are using a gun, you will be force-switched to the random plasmid every once in a while. You'll need another dosage of Lot 192 to fully cure yourself. Head back down the elevator and leave the Mercury Suites. Return to Central Square and proceed through the Bulkhead to Apollo Square.