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, although it also destroys the force fields erected by the Enclave. However, the ill equipped Lyon's Brigade is massacred by a barrage of missiles and small arms fire, leaving the path strewn with the gruesome remnants of Enclave and Brotherhood of Steel soldiers alike.

When the player and the remaining Lyon's Brigade members finally reach the Project Purity control room, they discover Colonel Autumn and 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).

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.

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, his dialogue option has him refuse to enter the chamber, mysteriously claiming it is the player character's "destiny."

Whether this is because Fawkes believed himself unable to survive or if he had other reasons for declining the player (such as feeling his debt has 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.

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."

Choosing not to decide

Inexplicably, the doors of the rotunda will have locked, preventing the player from leaving. However, the player can also simply wait for the Purifier to explode.

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 enclaves. This would reverse a great deal of genetic damage done by the nuclear apocalypse, but at the cost of complete eradication of all life on the wasteland.

Game Over
There is no way to possibly continue playing after completing this quest. 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. The point of no return is right before entering the Jefferson Memorial rotunda.

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 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.

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.