You'll Know It When It Happens

 is a quest in Fallout: New Vegas.

Quick walkthrough
President Aaron Kimball is visiting Hoover Dam, prevent any assassination attempts.

Detailed walkthrough
There are three different assassination attempts:
 * 1) A sniper dressed as an NCR Ranger, who will appear atop the guard tower behind the stage where the president gives his speech.
 * 2) A bomb placed on the Vertibird by an engineer.
 * 3) A Legion soldier disguised as an engineer, who will run up to the President and try to kill him with a knife.

If you wish to complete the optional security sweep, there are two things to find: a bloodstain in the Supply Closet (requires 6 perception), and recent unauthorized activity on the computer terminal in the Visitor's Center (requires 50 science). While in the Supply Closet, you should take the NCR Engineer Jumpsuit from the locker. You can now tell Ranger Grant you're ready.You can also talk to a NCR member outside. He will let you borrow his dog for a security sweep if you have the animal friend perk.

If you received this quest from Yes Man or Mr. House, you will not be able to access the top of the Visitor's Center where the President's Vertibird lands unless you are idolized by NCR, disguised as an NCR member (this is where the NCR Engineer's Jumpsuit from the Supply Closet comes in handy) OR if you have a speech skill of 55. However, if you can access the landing area for the Vertibird, and you have 50 explosives or repair skill, you can disable the bomb. Wait until the Vertibird has landed and the President has left it, then "Activate" it and you will receive a message asking you to check for the bomb. Another option to defuse the bomb is to pickpocket the engineers until you find the man with the detonator. (You might be able to use a ladder to reach the platform. If you stand next to Grant and look at the platform, the ladder is on the left of the platform. Confirmed on PC and PS3 with neutral clothing only.)

Whether you have been able to defuse the bomb or not, you still need to stop the assassin at the guard tower. You will see him throw the real guard off of the guard tower and take his place (it's a very noticeable scripted event that will occur unless you're waiting atop the tower when the assassin appears, in which case he will attack both you and the real sniper immediately). Kill the assassin. It is possible to both defuse the bomb and reach the top of the tower in time, but it is not a foolproof method. The easier option is to defuse the bomb, then run and wait near the large anti-aircraft gun (though don't go behind the stage, as everybody will turn hostile if you do). When you see the Ranger thrown off the side of the guard tower, you know it is time to take out the assassin using your weapon of choice. If you are a good enough sniper, the assassin can be killed from a position atop the same building the Vertibird lands on. (Side Note) If you have Rex in your company he will detect the Legion engineer in the group, after talking to the engineer (the only one you can talk to) he will become hostile, take him out or let the Rangers do the job.

Another option is after dealing with the Legion sniper perform pick pocket on the Legion engineer to receive a redundant fail safe detonator, return this to ranger Grant this will trigger a cut scene where the rangers will intercept the Legion engineer whilst the president flees to safety.

The simplest way to deal with the sniper that does not require any weapon skills at all and leaves you in the area to deal with the engineer is to simple climb the tower BEFORE starting the quest, drop a C-4 Explosive in front of the table, then head back down. It will remain there as you start the quest. Deal with the explosive on the Vertibird and watch the tower as the speech starts. You will see the assassin kill the Ranger and toss his body over the ledge. Pull out your detonator and BOOM. You will see the explosion and the assassin will be killed.

Another possible way to complete this mission is too go straight to the overlook and speak to Ranger Grant. Go up to the Helicopter Pad and wait for the Presidents Vertibird to arrive. Then an engineer will appear and place the bomb on the Vertibird. Interact with the Vertibird and choose to search it to discover the bomb. Either pick-pocket the detonator or disarm the bomb. Then run down and tell Ranger Grant. He will cut the President's speech short and a sniper will shoot at the president, miss and hit a Ranger. The president will then run to his Vertibird and escape. This is actually an easy way to obtain the NCR Ranger Combat Armor.

An alternate method to completing this quest is to forgo defusing the bomb (for example if going atop the Visitor's Center will turn the NCR hostile, or if you lack the sufficient repair or explosives skill), and wait atop the guard tower where the assassin will appear. Kill the assassin when he appears from a hatch in the corner, and then use the radio on the table to contact Ranger Grant. Letting him know of the attempt will cause him to cut short the President's speech and sweep the area again, causing him to find the bomb and defuse it. After you have radioed to Grant, go down by the stage, and watch for the (Legion) engineer who will run up to the President, and kill him if he does. The President will then escape safely.

Another alternate method to complete this quest is to have Rex as your companion when starting this quest. Talk to Ranger Grant, and tell him you need access to all areas. This will allow you to freely reach the roof of the Visitor's Center after you leave through one of the two double door sets leading outside to the dam. After you are outside, tell Grant that you have finished your security sweep, then proceed onto the roof. You should see a mechanic near the door to the stairway. Talk to him until he engages in dialogue, then choose the dialogue that includes your dog, Rex. After this, ask to go somewhere private. The mechanic becomes hostile, and you can kill him (or the Rangers will kill him) without penalty. Loot the failsafe detonator off his corpse and return to Grant. The quest concludes.

The easiest alternate method uses the fact that the engineers shouldn't have guns, and requires that you wear the engineer jumpsuit from the supply closet. First, pickpocket the failsafe detonator off the false engineer on the landing pad. Then when the Vertibird lands pull out a gun and the rangers will become hostile to you. Put your gun away and they will de-aggress and the President's vertibird will fly away without him ever getting off, completing the quest in the process.

(Optional)
Investigate the Area for Clues.
 * On the second floor of the Visitor Centre you can talk to Allison Valentine who is looking for her Engineer friend who is late for the President's Speech. His blood can be found in the Supply Closet in the Hoover Dam visitors Center.
 * Examine (Perception 6) a blood stain in the Supply Closet in the Hoover Dam visitors Center.
 * Hack the computer terminal in Visitor Center to discover an unauthorized access ID (Sgt. Macovitch).

If you have Boone as a companion sometimes he'll say something about if he were the sniper, where he would shoot from. He'll tell you 4 locations, three of which have assassins at them...

Bugs

 * If you start Wild Card: Finishing Bets after starting this quest, you won't be able to talk to Ranger Grant (only one dialog option - Goodbye) and thus unable to start the event. Your only option is to finish Finishing Bets and let the President die.

Quite a helpful Bug, on some occasions you do not have to take care of all three assassination attempts, in some cases only one assassination attempt foiled (The Bomb on the Vertibird) and the other attempts will become void and the mission complete.

After killing the sniper, report back to Ranger Grant. His first dialogue will be normal but his second sounds like it's coming through an intercom.

After completing the quest all NPC still act as though the speech was still going on and will not engage in normal dialogue, especially Moore.

If the player completes Wild Card: Finishing Bets and completes the optional objective to save the President, this quest will still be given, and Grant will be unresponsive, simply standing and gritting his teeth, rendering the NCR ending effectively impossible to finish. The Eureka quest will not be given and the instructions to go to the general will not be given.

Similarly, if this mission is gained through the House quest line, and then Mr. House is killed or disabled (as per an NCR request), then when Kimball is safe and the quest is marked as complete, the NCR troopers will not return to their positions and you cannot advance. This is particularly devastating, as it can effectively end the game depending on your previous choices.

If you cant speak to Colonel Moore after saving the President Kimball, open the console with ~ and type

This will allow you to tell the Colonel the president is safe.

It is possible that the President glitches and does not walk to the stage to make his speech, stopping the "Sniper event" from starting. A way to complete the quest despite this bug is to go atop the tower where a friendly NCR sniper is standing (the one who should be getting killed) and kill him. This will cause the President to flee and the NCR soldiers to shoot at you, but it will mark your quest as Accomplished and you will gain the corresponding NCR reputation as well. Simply escape the area and come back a bit later so the NPCs are friendly again to turn it in. Alternatively, you can perform a sneak attack with a grenade. Throw it towards the guard surrounding the president; however make sure not to be spotted. This will keep NCR friendly and complete the quest.
 * Another way to fix this (ONLY WORKS ON PC):
 * 1. Take up console using the tilde key (~)
 * 2. Click Colonel Moore, who should be standing in front of the president, and write
 * 3. Write
 * 4. Exit console

There is a very easy way to complete this quest, but it generally only happens by accident. Because the NCR Veteran rangers next to the stage will fire on you or your companions if you get too close, getting a companion near them at the right time can cause the quest to be completed. The timing is essential; you need to get your companion to walk past the rangers just as the Vertibird touches down. (This does not work on Hardcore mode) The rangers will attack and knock out your companion - it is imperative that you do not fire on the rangers. The Vertibird should take off immediately and a message will appear saying 'quest complete'.


 * It is possible that before reporting to the Ranger for the first time, many members of the audience will be stripped bare of all of their clothing and will stand there in their underwear during the speech.


 * It is possible to have one companion get stuck next to the Vertibird while running to take out the sniper, if this happens and you run past the President towards the Sniper, then the NCR Sniper near the Vertibird might kill the Legion Sniper for you, this is particularly helpful as the president will instantly flee, however you still have to climb up to the guard tower to trigger the end of the Quest.


 * Another Bug is sometimes one of your Companions will constantly try and talk to you during the president's speech.


 * In some cases the President won't be able to move because he falls through the floor or runs without actually moving, making the quest unable to complete.


 * If the game glitches and the president wont leave the stage after completing his speech (you must have killed the sniper) pickpocket the engineer near the Vertibird for his detonator, then make your way to the tower where the sniper was located, crouch down and take out one of the NCR Rangers.


 * The PS3's lockup bug where the machine freezes up after extended periods in the open wasteland is especially prevalent during this quest. Simply restart the machine and reload the autosave and re-attempt the quest to get around this.


 * Another way to get the achievement not complete the quest) is to; once the speech starts attack an member of the NCR, then before the president does, climb the ladder to the Vertibird and diffuse the bomb, the president will get in and take off netting you the achievement, but all three factions will now hate you and you will have no way to complete the game, so just reload and fail the quest to continue the game.


 * Theres a way to complete the quest without doing anything...(i used Cass for it. 360)first talk to Ranger Grant. then as the presidents vertibird is landing, just run along the right side of the fence toward the stage, jump over the fence and your follower will run behind the stage causing them all to attack your follower(knocked cass unconscious)this caused the vertibird to land and then right away take off. mission successful just report back to colonel Moore that the president has safely left. no need to defuse the bomb or anything.


 * An easy way to do this quest is to pull out your gun when Kimball's vertibird lands. Everyone should become hostile, then holster the gun. This will complete the quest and it seems you completed the quest normally. This will also get you the trophy. (PS3 Confirmed)

Behind the scenes

 * After killing the Legion sniper on the guard post and using the radio to report to Grant, one of the dialog options ("Everything's fine up here, thank you. How are you?") Is a reference to Han Solo from Star Wars episode IV: A New Hope when he and Luke Skywalker are saving Princess Leia from the Death Star.