Once inside, make sure to switch to a suppressed weapon. You can always go in guns blazing, but sneaking in is much faster.
When you enter, you will find two guards directly ahead of you. Quickly take them both out with headshots.
Now, you will have the option to take the elevator to the top floor or use the stairs. Do not take the elevator, but go through the stairs.
When you go up the stairs, you will find a single guard standing by the door to the next stairwell.
Do not kill this guard yet. There are two more guards through the door, one of which will come through the door and next to the standing guard. Wait for the second guard to enter the door and then take out both guards together.
Go through the door, and you will find another guard facing away from you. Take him out and continue going up the stairs.
You will find another guard on his phone on your way upstairs. Take him out as well.
Finally, you will find two more guards next to the entrance of the top floor. Take them both out and proceed forward.
On this top floor, you must search for clues that may lead you to Labrat. There are two key items you need to collect here. The first is a clipboard with Labrat's picture on it. The second is the basement keys.
Once you reach the top floor, wait for the patrolling guard to return to the right side and take him out.
After taking the guard out, continue moving forward from the right side. Go through the office to the right and kill the scientist inside. Make sure to keep an eye out for the clipboard with Labrat's picture on it — it's on one of these desks or tables. The location is always random, but it will always be inside one of the offices on the top floor.
Continue to the next office ahead of you and eliminate the scientist inside. This is where we found the clipboard with Labrat's image during our playthrough.
Continue moving forward and enter the middle office. You will find one scientist inside here. Take him out silently as well.
Exit the middle office through the middle door. Do not continue going forward through the right side. Exit through the middle and turn right.
Wait for the patrolling guard to come outside of the next office and take him out.
Go inside the office, take out the scientist who is standing furthest away from you, and then shoot the one near you.
Fortunately, the keys to the basement were also on one of the tables inside this office. Again, it can be completely different for you.
Once you've collected the keys, make your way back to the stairwell and go all the way to the bottom. You will face some guards on your way down, but they're not too much of a hassle. Simply take them out and continue going down.
You will meet Luchadora at the bottom, next to the locked basement door. Unlock the door using the keys and enter the basement.
Inside the basement, you will come across some more enemies. Take them out, and a cutscene will play.
After the cutscene, you will be in a car, getting chased by the enemies. Drive back to the Freakshop to complete the mission.
With Labrat rescued, the next step is confronting Dr. Friedlander himself in Checking In.
