The final mission
This is the final mission, after which the credits will roll and you'll no longer be allowed to freely roam the world. You'll have to start from a saved game or from the beginning. Therefore, we recommend completing all side missions and stocking up on ammo and medical supplies first.
Having gathered our courage, we head to the lab and declare our readiness. After we speak, Rothschild activates a huge robot that will assist us in the final battle with the Enclave. We exit the citadel after our iron friend. The robot is immortal, so there's no need to worry about it, and neither should we press forward. At first glance, it might seem like all the bombing around is scripted. It is, in part, but scripts can bite too. Simply follow the robot obediently (preferably from a distance) and help pick off the Enclave fighters.
Upon reaching the Jefferson Memorial, kill the soldiers on the left on the stairs and head inside. Guard Linos will be our immortal companion, so things are already easier. An Enclave squad awaits in the room to the right. We kill them with whatever comes to hand and proceed further to the rotunda. Inside, we'll run into Colonel Autumn. Speak with him. If your speech skills are high enough, we can convince him to surrender voluntarily. If not, we kill him and his two guards.
Just when it seemed we'd won, Dr. Lee delivers some sad news. If we don't activate the purifier soon, the entire structure could explode. Radiation levels inside the chamber are off the charts, so whoever activates the purifier will die. We're faced with a final choice that affects the game's ending (along with other decisions throughout the story):
1. Sacrifice yourself, enter the chamber, and enter the code (2 1 6), after which we die and watch the final cutscene.
2. Send Linos into the chamber and watch her die, after listening to a lecture on heroism.
3. Wait a bit and let the Memorial explode.
In the first two options, you can also insert the flask containing the virus given to you by President Eden. To do this, activate the additional filtration panel and install the infection. But remember, half of all living beings, even those slightly irradiated, will die.
This concludes the story of the guy from Vault 101. Congratulations on completing the game!
Source: SSoHPKC
See also:
Companions in the game (part 1)
Fallout 3: Tempenny Tower
Location Map (with bobbleheads)