Take it Back!

 is a Fallout 3 quest. It is also an Xbox 360 and PC achievement. Take It Back! is the final main quest, with the game ending immediately after its completion. In this quest, the player and the Lyons' Pride assault the Jefferson Memorial in hopes of activating Project Purity. A colossal, crude, yet brutally effective robot paves the way, annihilating numerous Vertibirds and Enclave soldiers, as well as the force fields erected by the Enclave.

The point of no return is right before entering the Jefferson Memorial rotunda. If you choose not to enter the rotunda, you may still exit Jefferson Memorial and map out.

When the player and the remaining Lyons' Pride members finally reach the Project Purity control room, they discover Colonel Autumn and a few other Enclave troops waiting for them. Player characters with high Speech skill may convince Colonel Autumn to step aside. Others may (rather easily) dispatch the Colonel after a fruitless conversation.

At this point, the player is informed over the intercom that Project Purity has sustained damage, possibly deliberate or accidental, and is about to self-destruct. Contrary to the more open-ended nature of the rest of the game, the player is now forced into two decisions.

Decision 1: Self-sacrifice or sacrifice another.
Damage to Project Purity has resulted in an incredible radiation buildup that will overwhelm anyone who is in the chamber during activation regardless of whatever preventative measures they have taken (e.g. Rad Away, Rad-X, or Radiation Resistance).

If the player hasn't figured it out from James' notes and recordings, the purifier password is 2-1-6 (referring to the verse from Revelations which Catherine loved. It is not specifically given as the activation code at any point in the game. The player is supposed to infer it from James' dialogue about the Lone Wanderer's mother, and the picture hiding his safe in the Vault 101 clinic. This can be verified by forcing Sentinel Lyons to start the Purifier (your character tells her the passcode in dialogue).

There are only two choices of who activates Project Purity: the player character or Sentinel Lyons. If the player asks any other companions, the dialogue option will be there, but they will refuse. Strangely, even 'good' karma allies who would not be harmed by the radiation refuse to assist the player.

Fawkes

Fawkes, one of the companions the player may take with them, has great radiation resistance (or possibly immunity) as a result of being a super-mutant. However, like any other companion character, her dialogue option has her refuse to enter the chamber, mysteriously claiming it is the player character's "destiny."

Whether this is because Fawkes believed herself unable to survive or if she had other reasons for declining the player (such as feeling that her debt had been repaid) is not explained. Regardless, Fawkes turns out to be a red herring: the player is forced into a decision to either sacrifice themselves or make a true hero (Sentinel Lyons) make the sacrifice for them.

Fawkes may, however, follow the player into the chamber! (And they said she wasn't evil!)

Dogmeat

In addition, a new dialogue option with Dogmeat opens up, wherein the lone wanderer can say some final words to his canine companion. As intelligent as the canine may be, there is no option to have Dogmeat activate Project Purity.

Sergeant RL-3

Even if you took Sergeant RL-3 (Mister Gutsy combat droid, which is immune to radiation) into the rotunda with you and command it to go in to turn on the purifier he "declines" and says "Soldier, you ought to be ashamed of yourself. This is your fight. You gotta finish what your daddy started. Stand strong and get your ass in there."

There is no Robotics Expert dialogue option in the conversation. So much for the 3 laws of robotics.

Charon

Although he presumably could survive the radiation like Fawkes (and he will retrieve the G.E.C.K. from Vault 87 if requested), Charon simply says that "[he] saved your ass plenty of times already."

Slightly more predictable, given his murder of his previous master.

Butch

Butch, understandably, will refuse out of fear of dying and of becoming a ghoul.

Star Paladin Cross

She refuses of course: "My friend... It's been my pleasure accompanying you, but we both know this is your fight to finish. Stay strong and honor your father's memory."

Clover

Apparently, there are limits to Clover's mental conditioning and slave collar. She refuses as well:

"Honey, you are out of your mind if you think I'm going in there. Find yourself another guinea pig."

Jericho

(Missing) Well duh he said he was "a real ass hole"

Choosing not to decide

After you enter the rotunda, Lyons will comment of locking the area down and both doors out will be sealed (not locked, and as such unpickable via lockpicking and the console), preventing the player from leaving, although a companion fired at this time is still able to depart through these locked doors. However, the player can also simply wait for the Purifier to explode. There will be at least one audible warning from Dr. Li. Remember, the code to the purifier is 2-1-6-ENTER.

Decision 2: Introduce the modified FEV virus.
If the player still has in their possession the modified FEV virus created by President Eden, they can insert it into the water filter. As in Fallout 2, by design the only creatures spared by the modified FEV virus are pure strain humans who are still within vaults or the Enclave. This would reverse a great deal of genetic damage done by the nuclear apocalypse, but at the cost of complete eradication of nearly all life on the wasteland.

Game Over
There is no way to possibly continue playing after completing this quest, short of a hack or modification. Whether the player chooses to go in there themselves or make Lyons' Elder's daughter do it, the game ends. The ending cinematic plays and the player is returned to the main menu.

If the game did continue at this point, the resulting changes may have lead to a less-than-satisfying result. For example, if Project Purity activated successfully, all water sources connected to the Potomac river should no longer be irradiated, resulting in free healing throughout the game, rendering stimpaks and consumables such as Dirty Water or Nuka-Cola nearly pointless. Also, if the player had introduced the modified FEV virus, everything would be dead (or dying) with the possible exception of those within Vaults. It remains to be seen which result is canon in the March 2009 "Broken Steel" downloadable epilogue.

Modifications/hacks
For PC users, one mod exists to play after the ending sequence, regardless of the choice made (even if the player sacrifices himself). It is found here. Because this is unofficial however, none of the above changes happen after the game, though there is an included module for a purified Potomac.

Future Expansions
The first two sets of Fallout 3 downloadable content from Bethesda (Anchorage and The Pitt) will be prequels to the events of Fallout 3. The third DLC, scheduled for March, will be a continuation of the Capital Wasteland storyline. There is no news yet as to whether the player will have to create a new character to access these scenarios, but given only one possible Fallout 3 ending permits the character to survive (have Lyons enter the Purifier), it seems likely that a new or heavily modified character will be required to play, unless the assumption is made that the character chose the "cowardly" path. Though it is possible they may "ghoulify" the player, however unlikely. It is also possible the ending will be tweaked at this point.

According to the Bethesda press release, the next three monthly DLCs will be:

Operation: Anchorage. Enter a military simulation and fight in one of the greatest battles of the Fallout universe – the liberation of Anchorage, Alaska from its Chinese Communist invaders. An action-packed battle scheduled for release in January.

The Pitt. Journey to the industrial raider town called The Pitt, located in the remains of Pittsburgh. Choose your side. Scheduled for release in February.
 * If you spoke with Outcast Protector Casdin, he discloses that he and the elder Lyons cleansed The Pitt on their way to D.C., suggesting that this is either a prequel or it was not fully purged.

Broken Steel. Join the ranks of the Brotherhood of Steel and rid the Capital Wasteland of the Enclave remnants once and for all. Continues the adventure past the main quest. Scheduled for release in March."

Ending Movies
The ending movie varies depending on how the player plays the game, and some of the largest differences are seen depending on what the player has done during Take it Back! See Fallout 3 endings for more info.