The Velvet Curtain

 is a Fallout 3 quest that is added to the game with the Point Lookout add-on.

This particular quest leads you in an investigation to find out the whereabouts of missing Chinese agent, Wan Yang, and use the Submarine Self-Destruct Codes in her possession to blow up the stranded Chinese submarine SSN-37-1A.

Walkthrough
To start this side quest, travel to the homestead motel, north of Pilgrim's Landing. Enter Room 1D and activate the terminal. You will receive instructions to go to the lockers that are south of the motel and use a key to get into the locker. The key is located inside the briefcase on the bed. You can also start this quest by using a terminal in Turtledove Detention Camps Administration building, and reading about the spy alert.

Obtain the orders from the locked locker
Once at the locker (Located on the right side of the boardwalk coming from the ferry), open it and take the holotape Box 1207 - Spy's Audio Password. The holotape instructs you to go to the bank and use the password on the tape. It also informs you the safety deposit boxes can only be accessed by their voices.

Access safety deposit box 1207
At the People's Bank of Point Lookout, go to the the safety deposit box room and stand in front of the speaker. Activate the holotape you received from the locker. The speaker will say "Safety deposit access granted. Have a pleasant day" upon which box 1207 will open. Pick up the holotape "Espionage Debriefing". This tape informs you of Agent Jiang's mission to destroy a captured Chinese submarine and his search for another agent, Agent Yang. Should you tell the speaker the right password and it does not unlock the safety deposit box, you must play the holotape from your inventory while in the room to unlock it.

Find Agent Yang
Agent Yang has the codes to destroy the sub within a false molar. The molar, unsurprisingly, is in her head. Exit the bank and walk to the Recruitment Station. Go through the first door on the left and activate the DiA Officer's Terminal. Access the file "Field Report - Yang Capture/Transfer". Read it to learn the fate of Yang being captured by U.S. officials and detained at Turtledove Detention Camp.

Turtledove is directly north from the Recruitment Center. The camp is devoid of carnivorous Hillfolk, but swarming with defence robots and ghouls. Enter the morgue through the cellar door, open Yang's remains locker and take the Submarine Self-Destruct Codes. You may escape the morgue the way you entered it (through cellar door), or you may use the hatch located near the cremation chamber, which leads to the Septic Tunnel. In the Septic Tunnel, you may pick some loot and exit to the east of the Turtledove Detention Camp, in the swamps.

Also, the Septic Tunnel might be a good way to enter the morgue without fighting minions of Ghouls and Sentry Bots - investigate the area to the west of Turtledove Detention Camp, two or three squares away, while periodically checking your Pip Boy's local map (as the entrance is not displayed at the world map) near the Trash Heap.

Blow up the Chinese submarine
The Chinese spy sub is underwater, to the south of the Calvert Mansion and near a pair of sunken vessels, or more notably just west of the lighthouse. Enter the sub through a hatch on the comm tower. Use the terminal in the bow to activate the self-destruct, and flip the failsafe switch in the stern. Before you flip the switch, however, use your Lockpick on a trunk, which contains several valuable items, including a Power Fist. You'll have about 30 seconds to escape after leaving the sub, but you can stay inside for as long as you wish.

Return to the motel
Head back to the motel, and receive your new orders from the safehouse terminal. You'll be asked to meet the head Chinese agent at his bunker, located under Calvert Mansion. The terminal directs you to a pair of Cryptochromatic Spectacles hidden within the toilet tank in the motel.

Head to the bunker
Travel to the Calvert Mansion and enter the steel skeleton of a green house to the west. Equip your Cryptographic Glasses now. The number of rings on each vase represents its number. Using the Cryptochromatic Sequence activate the vases. The sequence is 1_3_2_3_4_4_2. A hatch will appear on the floor next to the # 2 vase. Inside, you'll notice a large amount of weapons and ammo, but will be beckoned by a Protectron. Give the robot the secret passphrase, and the bot will escort you to a room downstairs for your extraction orders. Don't follow the Protectron right away. There are numerous crates with various types of ammuntion, and a Stealth Boy next to the door you came in, and a Stealth Boy and Mini Nuke on the shelf next to the doorway the Protectron leads you though. Enter the room, and pick up the Backwater Rifle on the shelf, next to the door, then activate the terminal. The terminal tells you that the funds for completing the mission have been wired to your next of kin and advises you to take your death honourably. The chamber will fill with radiation. To escape, you'll need to scale the nearby lockers and crates to a run-off tunnel or you can repair a button to the right of the door to unlock it. You can shut off the radiation valve with a repair of 30, and can open the door with a science of 71.

Entering the run-off tunnel or escaping through the door will complete the quest.

Reward

 * 350 XP
 * Backwater Rifle
 * Cryptochromatic Spectacles