He can be found sitting in a chair in Moriarty's Saloon before the start of the The Power of the Atom. If the player accepts the fusion pulse charge, he will go back to Tenpenny Tower once the task of re-arming the bomb is done and wait for the player to rendezvous with him. Alternatively, if the player disarms the nuke, he will leave Moriarty's Saloon and travel to Tenpenny Tower (this might take him a few days, and he may be killed in the wasteland).
If the player has the Black Widow perk, she can seduce Burke. When seduced, he will refer to her as his lover and plan to go to Tenpenny Tower to convince Tenpenny to not blow up Megaton. Although he travels to Tenpenny Tower after leaving Moriarty's Saloon, he will still send four love letters to Colin Moriarty to give to the player. In the last one he will finally break up with the player.
If you refuse Burke's offer or disarm the bomb without speaking to him, hitmen will show up at random encounters and try to kill you.
If the player accepts the offer but reports Burke to Sheriff Simms, Simms will attempt to arrest Burke, but get shot. The player can intervene and save Simms, but this can only be accomplished by killing or disarming Burke before he opens fire, or by attempting to speak to Simms while he is confronting Burke.
If the player has helped Roy Phillips take over Tenpenny Tower before meeting Burke at Megaton, Phillips will make Burke his right-hand man.
If you kill Burke, you gain positive Karma, but can still detonate the bomb if you have already accepted the offer.
Mr. Burke has a house in Megaton next to the bomb in the middle of town, but he never goes there and does not even possess its key.
Mr. Burke will not be killed if the player sides with Roy Phillips during the Tenpenny Tower quest.
Mr. Burke and Roy Phillips will have a special non-player character to non-player character conversation on Tenpenny's balcony during which Mr. Burke reveals his intentions to blow up Megaton and Phillips expresses his shock if the player decides to rig the bomb in Megaton and side with Roy Phillips during the Tenpenny Tower quest.
Mister Break is a character found in the Sacred Bog area of Point Lookout. He is a part of the hallucination that occurs after the Lone Wanderer consumes punga seeds and is seen leaning against an atom bomb, similar to the Megaton bomb but much larger. After interacting with him, the bomb explodes, and the Lone Wanderer wakes up outside of the Sacred Bog. Mister Break is a delusional image of Mister Burke. His clothes are the same as Mr. Burke's. When spoken to, he will respond in Tobar the Ferryman's voice (it is later revealed that it was indeed Tobar talking to the player), and he gives a warning to not get up, saying "you might hurt yourself." Mister Break is an essential character.
Mr. Burke does not exist in the Japanese version of Fallout 3, thus disabling the "detonate the bomb" option in The Power of the Atom as the player cannot get the fusion pulse charge under any circumstances, which was done for cultural reasons.
ps3 If the player kills Burke, after a long while he may respawn, and can be found standing beside the bomb if it is disarmed, though he cannot be engaged in conversation.
ps3xbox360 If you kill Burke and disarm the bomb, you will still encounter hitmen. If you loot one of their bodies, you will find a contract instructing them to kill you, signed "-B", as though you hadn't killed Burke.