In this page you can find a walkthrough for Who is Decemeber? side mission found in Prey. This is one of the more complex side missions and you can activate it during one of your visits in Talos I Lobby. During this quest you must complete a set of tasks given by December and then reach the Alex Yu's escape pod so you can escape from Talos I station prematurely. Using Alex Yu's escape pod is connected to the achievement/trophy called Abandon Ship.
This quest should be automatically unlocked after you make enough progress in the main story. When you start Detour main quest in Talos I Lobby and move to the passage that leads to Psychotronics you will be contacted by a person that calls themselves December. You are asked to meet that person and discuss getting the keycard to Alex Yu's private escape pod that can be used to escape from Talos I station.
The meeting place is in Neuromod Division Lobby and when you get there you see December being destroyed by an operator controlled by January. You can listen to January's brief commentary. But the most important thing to do is to examine destroyed December. You find a neuromod and a note with the following code: 5717.
This quest can be continued only when, in the later part of the game, you reach the part of the station called Arboretum and use one of the possible methods (activating the Grav Shaft, climbing rocks) to reach Alexa Yu's Office. There, you must find the crate presented in the picture. You can move it by using Leverage II or, e.g. place an explosive item next to it and detonate said item. Alternatively, you can smash the glass wall of the Looking Glass. Inside of the small room you find a safe and you open it by using this code: 5717. In the safe there are, among others, 2 neuromods and a TranScribe but the most important finding is the Alexa Yu's Apartment Keycard which can be used in the Crew Quarters.
This quest can only be continued after you arrive at the Crew Quarters. Clear the main lobby from mind-controlled workers and aliens on the level 1. At the end of it you can find the Grav Shaft presented in the picture that leads to the Executive Suits. Sadly, the Grav Shaft is inactive which means that you must find a solution to this problem.
The first solution is to hack into the Grav Shaft's system but it has level-3 security measures which means that you need Hacking III ability.
The second solution is to find the password that unlocks the Grav Shaft. Unfortunately, the password is generated randomly but the notes with it are easy to obtain and can be found in a few places in the Crew Quarters. The best way is to reach the small room in the west part of level 2 where you can find a recycler. You can get there by using the stairs that are near the starting point but be careful because inside you can find a Phantom which can be accompanied by Mimics. Search the room and look for the note with a randomly generated password (the example can be seen in the picture above).
When you get to the Executive Suits on level 3 you must go to Alex Yu's Apartment (its number is 303) and use the keycard that you found earlier in the safe. Watch out for the Greater Mimic inside (and for the Phantom in the adjacent destroyed apartment). When you secure the area you should go to the bedside table. Pick up the globe presented in the picture. Under it you find the EP101 Keycard.
You must now go back to Alex Yu's Office located in the Arboretum (you can use the panel near the Grav Shaft or climb the rocks). When you reach your destination you must use Alex Yu's computer. It has level-4 security measures which means that you need Hacking IV ability in order to unlock it (you can return here later if you currently don't have this ability).
After hacking the computer you must select the last tab (Utilities) and select the option, presented in the picture, to extend the bridge to the escape pod.
Exit the office and use the stairs to get on the rooftop. You can now cross the bridge to Alex Yu's escape pod. Use the keycard found in Yu's apartment to open the pod. When you enter it you must close the door and use the terminal. Confirm that you want to start the pod and watch a short cut-scene. You are rewarded with Abandon Ship trophy/achievement.
Warning - The screen that appears after this "ending" is similar to the one displayed after your death. You can return to the last saved point (or select a save "manually") which brings you back on Talos I. You can continue your journey and unlock a main ending.