Fallout 3

Story Related, Unmissable

Do note that the description for this trophy is misleading because just finishing the "Take it Back!" Quest won't pop the trophy, you also have to finish another Quest you're given once you reach the Jefferson Memorial called "Project Impurity".

Make sure that before accepting this Quest you have made a manual save and backed it up somewhere, because this Quest is the point of no return if you don't have the Broken Steel DLC installed. You won't be able to complete any other Quests except this one once you start.

Note: On entering the Laboratory you will be unable to equip any weapons or use your Pip-Boy until the cutscene and conversation with Sarah are finished. When you finally tell Sarah you're ready, you won't be able to control your character again until Liberty Prime is being raised to ground level.

WARNING: Whatever you do, don't go through the main gate of the Citadel before Sarah does else there’s a chance you'll crash the game.

On exiting the Citadel you have to escort Liberty Prime to the Purifier but the very first time Harry did this Quest, Liberty Prime got his foot stuck on one of the rocks and he didn't get out for ages!

I just started wandering around the wasteland getting Bobbleheads and every once in a while I'd return and see if he was moving, most the time I checked he was still stuck then one time I killed some of the brotherhood (while being in a 'hidden' state) and carried the bodies to the foot of the bridge. To my astonishment, after doing that Liberty Prime started moving and I was able to complete the Quest.

Don't try shooting Liberty Prime if he gets stuck else he will turn hostile and kill you almost instantly.

The easiest thing to do is to just let Liberty Prime to do all the killing as he throws nukes and has a big laser for a face. The enemies that do get around him, you want to make sure you kill them quickly as their energy weapons will take a lot of health off you.

Once you reach the Memorial (and have triggered the Quest "Project Impurity") you have to fight off a few Enclave soldiers and when you reach the lab, Colonel Autumn and his goons will be there. Autumn will have a brief conversation with you then you can kill him and his goons, they don’t take much to kill. Once you've done that, approach the Purifier chamber and Dr. Li will say that the Purifier is going to explode if the pressure isn't released but it’s a one-way trip for whoever goes inside. Normally it doesn't matter who enters the code into the Purifier but sometimes it will glitch if your Companion enters it. If it does glitch then all you can do is load an earlier save and make sure only yourself or Sarah Lyons enters the code (credit to Gretty for confirming companions can start the Broken Steel DLC).

If you enter the Purifier then you have chance to use the Modified F.E.V virus (if you still have it) with the console next to the inner door of the chamber.

Without Broken Steel installed this trophy will pop either during or just after the final cutscene but with it installed, the trophy will pop sometime during or just after you speak to Elder Lyons when you wake up.

Note: If you activate the Purifier then there may be a minor but harmless audio glitch where Sarah will tell you to activate it even though you already have.