Holy Water (quest)

Holy Water is a Fallout 3 quest that is added to the game in the Broken Steel add-on.

Walkthrough
You pick up this quest from Scribe Bigsley after asking him what you can do to help. He asks you to investigate the area around Megaton to see why they need more water, after just being sent a week's shipment of Aqua Pura.

Investigate the vicinity of Megaton
In front of the entrance to Megaton you witness a 'cutscene' of sorts. An NPC named Rosa gives Micky a bottle of seemingly pure water. He drinks the water and dies. Rosa then approaches you, and after a short conversation she reveals that she received the water from a cult in Springvale.

Alternatively, you also receive various hints within Megaton, such as a note pinned to the door of your Megaton House or a NPC coming up to you and complaining about a cult stealing Megaton's water. The unnamed inhabitant of Megaton sitting next to the Brahmin at the bottom of the hill where you enter also gives you the Holy Water Pamphlet if you speak to him. If you have very good Karma then upon entering Megaton a resident will run up and give you the Pamphlet and complain about the cult.

Ask about Holy Water in the Holy Light Monastery
After finding the Holy Light Monastery, you encounter Brother Gerard. After giving you some Holy Water he refuses to speak to you further until you have drunk it. Once you have done that, he reveals that the leader of the church, Mother Curie III, has been "blessing" water so that wastelanders may "bask in Atom's Glow".

Gain entrance to the Monastery
There are five options on gaining access to the monastery. You may ask Gerard if you can meet Mother Curie III but you must either be irradiated (Equipping the Ghoul Mask will fulfill this option), pass a Speech check, picking the lock, simply killing Gerard and loot the key from his corpse or steal it from him via pickpocketing.

After gaining entrance, you enter the basement of the church by means of a trap door. Mother Curie is giving a sermon to two ghouls as you enter. Once she finishes the sermon she engages you in conversation. She tells you that she has been intercepting the Aqua Pura from the Brotherhood caravans (who she sees as "angels") and turning it into holy water. She does this because she wants to share Atom's gift with the people of the Capital Wasteland. This "gift" is ghoulification.

Stop the Apostles of the Holy Light
From here on out, you have three options. You can talk her out of stealing water with a Speech challenge or Charisma check, you can murder them all or you can irradiate yourself to above 800 rads. If you do the latter, Curie will think you are the Prophet of Atom and will willingly give up irradiating the water. You can irradiate yourself using the radiation traps (turned on with nearby switches), both in the basement and close to the entrance in Springvale. Be wary if you decide to kill them, as there is a Feral Ghoul Reaver named Atom's Champion and a Glowing One named Sun of Atom, both of which will turn hostile.

Killing them does not take away karma.

The Power of Atom
Should you destroy Megaton after undertaking the quest, when you return to Megaton Mother Curie III will be outside Megaton as a hostile Feral Ghoul Reaver, Brother Gerald will be a regular friendly Ghoul. He will say that he has seen Atom and that he is going to stay at Megaton to wait for Atom's guidance. Bigsley is shocked at Megaton's destruction but will still give you the money for completing the quest.

Bugs

 * If you don't settle the conflict outside of Megaton peacefully and Deputy Weld 'dies' in the gunfight, leaving the area and returning later may cause him to return to life.
 * The option to use the Speech skill to gain entrance to the monastery may not be available.