This quest is given to you by:

* The Sheriff Lucas Simms, who wants you to disarm the nuclear bomb in the middle of Megaton.
* Mister Burke who wants you to help him detonate it, as his employer, Allistair Tenpenny, considers Megaton a blight on the landscape of the Capital Wasteland; Burke can be found in Moriarty's Saloon in Megaton, casually sitting in the corner.

Detonate Megaton's atomic bomb
Rig Megaton's atomic bomb to explode

To do it, you have to put a Fusion Pulse Charge, given to you by Burke, inside the bomb. Your Explosives skill must be 25 or higher for you to attempt it; Mentats can be used to boost your statistic.

In the Japanese version of Fallout 3, Mr. Burke does not appear and the player cannot get the Fusion Pulse Charge. The only options are to disarm the nuke or ignore it.
Report to Mister Burke in Tenpenny Tower!

After rigging the bomb, meet Burke at Tenpenny Tower, which is southwest of Megaton near RobCo. There you can observe the explosion from a safe distance together with Burke and Tenpenny.

Mr. Burke will give player the privilege to push the button to detonate the bomb. Doing so instantly gives the player a Karma of -1000. If your karma before was 0 or negative, -1000 karma will instantly lead to the lowest possible karma in the game.

If you help Roy Phillips take over Tenpenny Tower in the Tenpenny Tower quest prior to completing Power of the Atom, you may still complete Power of the Atom. When you return to the top floor penthouse, you will stumble onto Roy Phillips and Mr. Burke having a conversation. Roy is intimidated by Mr. Burke and the fact that he made his way to the top floor. Roy agrees with Burke on the matter of blowing up Megaton and also states that he could use a man like Burke, thus Burke begins a new partnership with Roy Phillips.
Disarm Megaton's atomic bomb

Lucas Simms wants you to disarm Megaton's bomb. Your Explosives skill must be at least 25 to attempt this. Taking some mentats can temporarily increase it to the necessary level, unless it was very low to begin with. If you are successful you get a cash reward and the keys to your own Megaton House. The cash reward is usually 100 caps, but with a successful use of your speech skill on Lucas (before you actually disarm the bomb) you can talk him into a reward of 500 caps. If you do not mention payment, you will typically gain Karma instead, though if you disarm the bomb right away, without letting Simms welcome you to Megaton, he will give you the 100 caps.

Mr. Burke will leave Megaton after you defuse the bomb. As a result of disarming the bomb, Talon Company mercenaries hired by Mr. Burke will randomly appear in an attempt to kill you.
Optional: Deal with Mr. Burke

This section of the quest is entirely optional. You can kill Burke after speaking to him, without talking to Simms first, killing him will result in good karma and you can avoid risking Simms getting killed. You can also accept the quest to blow up Megaton, and then kill Mr. Burke, without penalty.
Optional: Leo Stahl's Drug Stash

If your Explosives skill is insufficient (below 25), you will be unable to rig or disarm the Megaton bomb. If your Perception is at least 5 and/or your Explosives skill is at least 20, you can raise it sufficiently by consuming Mentats. One nearby source of Mentats is Leo Stahl's drug cache, which is hidden inside a desk within the Water Treatment Plant. One can gain access via the Leo's Drug Habit quest, or by picking the lock on the desk (very hard).
Report Mr. Burke to Lucas Simms

After speaking to Burke, a new dialogue option will become available when speaking with Lucas Simms, which can result in him confronting Burke about the plot to destroy Megaton, as long as your Karma is low enough (if you got the Fusion Pulse Charge, mention it and Simms will take it away and proceed). Take care not to "cry wolf," so to speak, because he will not believe you if you rescind your initial warning. Lucas Simms will tell you to follow after informing you that you're about to see Wasteland justice first hand. Simms will attempt to arrest him. Burke will deny the allegations, but will eventually agree to be detained and suggest that Lucas "Lead the way." As Simms turns to lead Burke to jail, Burke pulls a silenced 10mm Pistol on him and kills him. (It seems that, if Simms manages to exit the room before Burke takes the shot, the latter will just shoot at nothing in particular. Lucas Simms can then be found dead, outside the saloon.) However, if you are quick enough, you can kill Burke as soon as he pulls out his gun. To do this, have a high powered gun (Scoped .44 Magnum, Sniper Rifle, Combat Shotgun or possibly even a Police Baton) drawn and ready while the two are having their conversation, and as soon as Mr. Burke draws his gun, blast him (preferably a headshot to ensure a kill) manually or with V.A.T.S.. If it takes more than 1 shot to kill him, Mr. Burke may still kill Simms. You can try to shoot the silenced 10mm out of Burke's hand instead of killing him. This will give the sheriff time to respond and give you more wiggle room in dealing with Burke since he will now be unarmed. If you kill Burke before he kills Lucas, the sheriff will thank you afterward.

If you don't act fast, Simms is killed. Afterward, Mr. Burke gives you a threat and disappears. If you kill Burke before he disappears, he says his friends will avenge him. If you let Burke kill Lucas Simms, then you kill Burke, you can loot both bodies. This will get you a Silenced 10mm Pistol and a Chinese Assault Rifle, plus some caps, ammo, and other assorted items. If Lucas Simms dies you can still complete this quest by talking with his son, Harden Simms, after disarming the bomb.

Neither option will result in a negative reactions from the townspeople, though they may comment "better him than me" if they see it take place. If in any way the player accidentally hits Simms, the town may become hostile and retaliate.
Seducing Mr. Burke

If you play as a female character and have the Black Widow perk you can seduce Burke and get him to spare Megaton. He will then leave the city, with the promise of sending for you later; he will even send you love letters, which can be picked up at Moriarty's saloon. Another possibility is persuading him to give you 1000 caps upon detonation of the bomb, as opposed to the 500 you would normally get.

Using the Black Widow Perk to seduce Burke still results in him putting a hit out on you, if you choose to disarm the bomb later
Reward
If you disarmed the bomb

* Bottle Caps
o 500 - If Speech was successful at start of quest
o 100 - If Speech failed, or wasn't employed at start of quest
* My Megaton House Key
* Deed: My Megaton House
* 300 XP
* Karma [+200]: If you disarmed the bomb for free
* A Contract for Extermination will be set out on you by Mr. Burke or Allistair Tenpenny; Talon Company will start hunting you.

If you destroyed Megaton

* Bottle Caps
o 1000 - If Speech was successful at start of quest
o 1000 - If the Black Widow perk was taken and unique dialog is chosen
o 500 - If Speech failed, or wasn't employed at start of quest
* My Tenpenny Tower Suite Key
* Deed: Tenpenny Tower Suite
* 300 XP
* Karma [-1,000]: If Megaton is destroyed
o With a lot of negative karma, the Regulators will start hunting you. You might also be ambushed by angry former Megaton citizens.

Walter's Scrap Metal refers to an unmarked and repeatable quest in Fallout 3.

After the pipes in Megaton are fixed, Walter offers to buy Scrap Metal from you to maintain them.
Reward

* Walter gives you 10 Bottle caps per Scrap Metal; if you decline payment, you will receive +10 Karma.
* You also get a 5 XP reward per Scrap Metal you give Walter.

Notes

* You cannot pickpocket Scrap Metal back from Walter, unlike in The Outcast Collection Agent. It is not even on his corpse.
* If you have XP bonuses (from Well Rested and/or Swift Learner), you should note that the game rounds the bonus up. Thus, with a 10% XP bonus, you will receive 11 XP for all "pairs" of Scrap Metal (because (2 * 5) * 1.10 = 11), and 6 XP for an "odd" piece of Scrap Metal (5 * 1.10 = 6, since it rounds up).
* Scrap Metal can be found throughout the Capital Wasteland and can be purchased from various Merchants.
* This can be a very lucrative ongoing source of income as the game progresses, considering that destroyed robots will occasionally provide scrap metal when looted. Fort Constantine, the National Archives, National Guard Depot, RobCo Facility and the Robot Repair Center can all yield potentially large amounts of scrap metal.
* If you have The Pitt add-on, a better use for the scrap metal might be to use the Ammo Press in the Mill.

Megaton

Objective: Investigate the nearby town of Megaton for information about James.

In Megaton, everyone will point you to the saloon owner, Moriarty who will offer to tell you where your dad is if you give him 100 caps. If you do not have the caps Moriarty will charge you 300 the next time you talk to him. You can also use speech to convince Moriarty that your father told you about him. If you choose certain dialogue options then he may offer a discount if you agree to kill Silver and bring back the caps she stole.

You can find Silver northwest of Megaton at Springvale, inside a ranch house. There are seven ways of dealing with her:

1. You can kill her and obtain 400 caps along with her equipment with NO karma change unless you start taking stuff from around the shack. She will attack you straight away if you say that you are there for the caps.
2. With a successful speech attempt you can convince her to give you 400 caps to tell Moriarty she's gone.
3. You can convince her to give you 300 caps to tell Moriarty she's gone.
4. You can ask for no money and tell him she's gone. This will result in positive karma, though you are still going to have to come up with 100 or 300 caps.
5. You can simply say she's gone without working out a deal with her, though again, you will still have to come up with the money on your own.
6. With the speech check you can make her give you 400 caps. You can then kill her to loot another 100 caps from her body. You'll lose karma if you kill her after she gives you the 400 caps though. However, donating 30 caps back to the Church of Atom will result in slight karma gain.
7. You can ask for no money and tell him she's gone. This will result in positive karma, you can then kill her for 400 caps loot along with her equipment. Also there will be no change to your karma.

If you choose the bad karma way to interact with Moriarty, you can steal the password to his terminal out of a locker next to it or you can pass a speech check with Gob, that will tell you about Moriarty's terminal and if you ask him about the terminal after the speech check he will tell you that Nova has the password, than you can try to pass a speech check or use Lady Killer (male character) to seduce into giving you the password, or simply kill him and loot the password off his dead body (make sure you speak to each named person in his saloon to obtain side-quests). However you obtain the information, it leads you to Galaxy News Radio. Getting to GNR is no cake walk, so make sure you level and gear up before continuing this quest.
Galaxy News Radio

Head to Super Duper Mart and from there to Wilhelm's Wharf, where you will meet an old lady who has some food for sale. Somewhere along this route, you will meet Bryan Wilks (if you haven't already) who begins the Those! quest. Cross the river (swimming the radioactive river is not advised) over the intact bridge found not far away. You will run into super mutants and raiders. Head for the first Metro Station you can find (for instance Friendship Heights or Farragut West Metro Station) to begin the long trek through the metro tunnels, expect to fight a ton of raiders and feral ghouls. If you follow the tunnels correctly, you should exit in Chevy Chase. The round dots in your Pip-Boy 3000 world map indicate intermediate points that are recommended, specifically, Metro stations, but you don't have to follow them, per se - just get to the goal, one way or the other.

From there you will eventually end up at Galaxy News Radio, where you can follow a group of Brotherhood of Steel soldiers (Lyons' Pride), led by Sentinel Sarah Lyons in an assault on the super mutants, super mutant brutes, and super mutant masters attacking the GNR building. The boss character for this assault is a super mutant behemoth. It is best advised to pick up the Fat Man nuclear launcher off the dead body of the Brotherhood soldier in the central fountain (there is a map marker pointing to it) as this will heavily damage the behemoth (make sure you aren't standing too close as this weapon has an extremely large blast radius, a good idea might be to run and grab the Fat Man then run up the stairs where Colvin is and use the Fat Man from there). Regardless, the marker still points to the dead soldier with the Fat Man, so loot him for the optional points.

Depending on how fast you take the behemoth down and who dies in the process, your conversation with the remaining soldiers goes differently. Use the intercom to the right of the door to get inside the GNR Building. Once inside, talk to Three Dog. You have two options at this point. If you have high enough speech, you can persuade him that your father will aid his cause upon your rescue. Doing this will give you the Scientific Pursuits quest for free. However if you have insufficient speech, he will give you the Galaxy News Radio quest, which you will have to complete first to get Scientific Pursuits.

If you used speech successfully, make sure to talk to Three Dog again. He will offer to exchange the location of a large cache of military munitions in return for the Galaxy News Radio quest. This allows you to explore the quest for greater returns.
edit The Search Continues

Killing Three Dog without getting him to tell you to go to Rivet City ends the GNR quest and starts a new quest called The Search Continues. The objective is still to find Dad, your father can still be found at Vault 112 and you can still talk to Dr. Li. You do not get an achievement because it might be considered an "unmarked" quest, even though you can put it as an active quest on the Pip-Boy 3000.
Skipping this quest

Note that this quest can be fast-tracked by skipping several steps:

* Visit Three Dog without speaking to Moriarty.
* Retrieve the Project Purity Journals from the Jefferson Memorial.
* Go directly to Doctor Li in Rivet City.
* Travel to Vault 112 where James is imprisoned.

If you end up finding Vault 112 before visiting Three Dog at GNR Plaza, this entire quest will be marked as completed. If you make it to the building, you'll still encounter all the super mutants (including the behemoth) with no Brotherhood of Steel soldiers in sight. The door to GNR will be locked but if you use the intercom after the mutants have all been killed, the Brotherhood of Steel soldiers stationed inside will let you in. You can then speak to Three Dog and get the achievement for Following in His Footsteps. Additionally, he'll offer you the Galaxy News Radio quest in exchange for the key and coordinates to Hamilton's Hideaway.

If you take this quest as normal and choose to find out about Dad on your own, eg. from Doctor Li or from Vault 112, returning afterwards to place the satellite dish, Three Dog will remark on how you didn't need to do as he asked, as you already had what you wanted. You will receive good karma and the key to the weapons cache at Hamilton's Hideaway for your troubles and for "fighting the good fight".
Notes

* Be very careful with the Fat Man, its immense damage may turn the BoS soldiers at the battle hostile.

* Be sure to loot the bodies of any fallen BoS soldiers. Their armor may be in bad condition, but it can still be used for repairs later on, sold, or given to Casdin as part of The Outcast Collection Agent quest.

* If you do use the Fat Man you may hear the surviving BoS soldiers commenting about it.

* Also if you use the Fat Man, be aware that anything you leave on the ground so you can move faster (such as the weapons from the downed super mutants and Reddin's Power Armor) will be thrown to oblivion and you'll never be able to find it if the items are within range of the launcher. Leave your heavy items in a nearby locker or simply refrain from looting corpses entirely until the Behemoth has been dealt with.

* Make sure to keep the holotags of any fallen BoS soldiers you find, as they will come in handy when you gain entry to the Citadel.
* If the player speaks to Dr. Li, enters the Jefferson Memorial, or rescues James before visiting Galaxy News Radio, the Lyon's Pride will not spawn in Chevy Chase, therefore the Lone Wanderer must fight the many super mutants on their own. The BoS knights still guard the GNR building but the Fat Man wielding brother will not spawn, and therefore the 8 mini nukes are unavailable as well.
* In his GNR broadcast, Three Dog will explicitly mention James' arrival at GNR before the player has learned this from Moriarty. However, learning about this from the radio is not sufficient to advance that portion of the quest and will not mark Galaxy News Radio on the map.

Find the Museum of Technology

You can ask Three Dog for directions to the Museum of Technology and the Washington Monument. From the radio studio, where you met Three Dog, proceed downstairs to the back door of the Galaxy News Radio building. From there you will need to locate the entrance to the Collapsed Car Tunnel. From the back door of GNR, you can see a bus below and to the right. The entrance is to the right of the bus. You may also notice some feral ghouls roaming around the bus. Cross the bridge on the left and turn right to find some loot at the picnic table, then you can jump down onto the bus.

Enter the Collapsed Car Tunnel door and follow the tunnels to Dupont Station. Enter Dupont Station and follow the waypoints south, while staying on the Red Metro line. You will pass the Central Station and then later reach the Museum of Technology. During this trip you will encounter lots of raiders, feral ghouls and a few super mutants, so make sure you have plenty of Stimpaks, ammo and weapons. See Metro for detailed directions.
Retrieve the Communication Relay Dish

Inside the Museum of Technology you need to find the Virgo II Lunar Lander, which is located in the West Wing of the building. You can obtain this information from a terminal in the Atrium to the right of the entrance when you come in. Note however that the place is crawling with super mutants. Your goal is to reach the West Wing going through the Vault-Tec demonstration vault, upstairs. You can also attempt retrieving Jiggs' Loot using the Museum Information Terminal in the Atrium.

In the West Wing, locate the Virgo II Lunar Lander. The west wing is divided in 3 areas: The Planetarium, the Delta IX Exhibition and the Virgo II Exhibition. Be ready for a fight as all these areas are patrolled in some way by super mutants. You need either Science 25 or Lockpick 50 to get through the Planetarium. So if you do not have those stats it's recommended that you go through the Delta IX Rocket Exhibit.

Once you've found the Lander and cleared all hostiles, grab the dish by interacting with the Lander and make your way to the exit.
Find the Washington Monument
The mounted dish.

Exit the Museum and head west for the Washington Monument. It is also in The Mall area and can be seen from the entrance of the Museum of Technology as a giant obelisk. There are Brotherhood guards stationed there and you'll need to use the terminal to open the security doors. The password is given to you in your notes at the beginning of the quest. Take the elevator to the top and interact with the radio equipment to install the dish. Finally, return to Three Dog at Galaxy News Radio to finish the quest.
Rewards

* 400 XP
* +200 karma upon installing the dish
* Information about James' whereabouts
* You can receive an additional reward if you finish this quest after discovering your father's whereabouts from someone other than Three Dog, or by using a successful Speech check to have him tell you where your father is. In addition to good karma, Three Dog compliments your dedication and then gives you a note titled "The Weapons Cache", the key to the weapons cache and the map marker for Hamilton's Hideaway where the cache is located. The cache is behind a cell door in the northwestern-most area of the hideaway, behind some rubble. It contains a Guns and Bullets, a Mini Nuke, a Stealth Boy, a leveled helmet and armor as well as other random loot.
Notes

* Even though you are led to this quest through the main story line, it is one of the quests that can be skipped. You can retrieve the Project Purity Personal Journals from the Jefferson Memorial, go directly to Doctor Li at Rivet City, or even straight to Vault 112 in the basement of Smith Casey's Garage, where James is imprisoned in a pre-war simulation, thus bringing about the quest Tranquility Lane. If you do elect to skip this quest, then you can always return to Three Dog and ask to help with the 'Good fight', and he'll offer you the quest.
* After you replace the satellite dish, the Galaxy News Radio signal increases in quality and can be heard all over the map. Tune into GNR and, shortly after you fix the dish, you'll hear Three Dog talk about the improved signal.
* It is possible to take an alternate route to the Museum of Technology. Instead of making your way through Dupont, head south, starting with Grayditch. Keep heading south until you reach Alexandria Arms. (At this point, your map should have determined your plan and rerouted the waypoint marker) Then, head east until you reach Rivet City. Next to Rivet City is the Anacostia Crossing Station. Take this station to the Museum of Technology. This will basically cut a lot of walking through stations off of your commute.