Underground Undercover is a main quest in the Railroad faction storyline in Fallout 4, and it involves helping the Synths within the Institute stage a rebellion and escape.
There are a lot of steps to this quest, so we will be looking at what you need to do to complete this Fallout 4 mission and claim your reward.
Underground Undercover Fallout 4 quest guide
1) Speak to Desdemona before you teleport to the Institute
The quest begins when you have done all the work necessary to create the teleporting device to enter the Institute. Desdemona will tell you about Patriot, the Railroad’s person inside the Institute who has been helping the Railroad get Synths out of the organization.
Talk to Tinker Tom, and he’ll give you an encoded holotape that can be uploaded to any terminal in the Institute to contact Patriot. Once you have obtained it, step onto the teleporter pedestal to transport to the Institute.
2) Upload the holotape
Interact with any Institute terminal and load in the holotape called “Network Scanner.” You’ll then receive a message from Patriot and a marker on your Pip-Boy will direct you to an in-person meeting with Patriot in the maintenance room in the Advanced Systems area.
3) Meet up with Patriot
Patriot’s real name is Liam Binet. He’ll lead you to his Synth accomplice, Z1-14. They will formulate a plan to rescue 13 Synths, but you’ll need a Pre-War username and password to bypass the security. Liam asks you to discuss the issue with the Railroad; you can then leave the Institute and return to the Railroad HQ.
4) File your mission report on P.A.M.’s terminal
At Railroad HQ, speak to Desdemona. She’ll direct you to the modified Assaultron P.A.M. and her terminal to input a report. P.A.M will then direct you to Cambridge Polymer Labs and the login details of a worker there, Wilfred Bergman.
5) Go to Cambridge Polymer Labs
To start your search for Bergman’s login credentials, go to the Cambridge Polymer Labs laboratory area. The terminal you need is on the second floor. The room is locked with an Expert-level terminal. If you can, hack it (you can use Nick Valentine if he’s your current companion).
If you don’t have Nick or can’t hack Expert terminals, go into the opposite room on the second floor where you’ll find a collapsed ceiling that acts as a ramp, allowing you to get into a space in the roof. Walk through the roof space until you can go no further and jump down through a hole beneath you.
6) Complete the research project
You’ll need to complete a research project side quest in the labs if you want to get out of the building, as a robot called Molly has locked the exits.
In the main work area of the labs, there’s a terminal with two slots on one side and one slot on the other side. You’ll need to find the correct two chemical agents and one isotope. You need lithium hydride (sample 3111) and gold (sample 611). The chemical agents aren’t too hard to spot, as they are in silver canisters.
The gold sample can be found in the same room as Bergman’s terminal, whereas the other sample is on the floor below. The isotope you need—U-238— is in an irradiated area that needs to be unlocked with a terminal. The Isotope Containment terminal is on the same floor as sample 3111, in the room next door. When this is done, head into the irradiated area to pick up the U-238 (the room is found to the right of the stairs you initially went up). Pop some Rad-X and have RadAway to hand before you head in.
Go to the terminal and insert the collected canisters on the left and the isotope on the right. You’ll earn the Piezonucleic Power Armor Chest, and Molly will let you out of the Clean Room so you can exit the building.
7) Return to Railroad HQ
Head back to Railroad HQ and speak with Desdemona. She’s concocting a bigger plan: She wants to try and free every Synth in the Institute. Desdemona wants you to cut Liam loose once you give him the password and work solely with his Synth accomplice Z1-14.
8) Go to the Institute
Fast travel back to the Institute and give Liam the password or speak to Z1-14. You need to do both, so order doesn’t matter. Z1-14 will agree to Desdemona’s plan, but he needs time to organize the Synths. You’ll be instructed to wait for 24 hours before speaking to him again. Choose a seat and press the prompted button to wait.
Once you have waited, speak to Z1-14 again, and he will confirm enough Synths are willing to get in on the plan. The main issue now is the group is unarmed. Z1-14 has a plan to take out some Institute guards and stage the deaths so the Synths can take their weapons. Since the Synths are yet to be armed, you’ll need to go down to the tunnels and take out the guards.
You can also go to the room opposite where you met Liam and transfer any weapons you want into a locker to aid with the Synth rebellion.
9) Kill the guards
Follow the Pip-Boy marker to find the elevator to take you down to the tunnels. Take out the guards and return to Z1-14.
10) Keep helping Father
Z1-14 will tell you to keep helping Father so suspicions aren’t raised. You’ll need to complete the Institute quest line up to the “Powering Up” quest; the final Institute mission. When you get to this mission, you’ll be tasked by Father to eliminate the Railroad.
11) Speak with Z1-14 for the last time
You’ll be prompted to speak with Z1-14 a final time, and he’ll tell you the Brotherhood of Steel has found the Railroad and is planning an attack. He asks you to warn the Railroad (it’s not a timed task, so don’t worry if you want to finish other missions first), so head to the HQ and speak to Desdemona.
12) Warn the Railroad
When you have warned the Railroad, the final quest of the game will begin, and you’ll have locked in your allegiance with the Railroad. The Underground Undercover quest will be complete, and you’ll be rewarded with the Railway rifle weapon.
