Eureka!

 is a final quest in Fallout: New Vegas.

Quick Walkthrough
Take over the Hoover Dam with the NCR.

After saving or not saving the President, you are asked to talk to the General and given the chance to turn back as this begins ending quest for the game. If you say yes, Legion members attack the Hoover Dam and you fight your way through the power plant rooms and head towards their base east. After fighting multiple difficult groups of enemies you are aided by other NCR Rangers. In the camp you fight Caesar's Legate, a very powerful champion of sorts that can be spoken out of fighting if your speech or barter level is 100. Once defeated, you meet up with the General and after the speech, the ending game scene then plays out.

Detailed walkthrough
You will start in Power Plant 04. Mike Lawson will speak to you. If you wish to follow his suggestion and do the optional part which involves flushing the Legion invaders through the turbines, agree to his suggestion. Through the rest of this, kill any Legion troops you may encounter along the way. Enter Power Plant 03.

(Optional) If you are completing the optional part, there will be a staircase immediately to your left leading back upstairs. Go up here. If your Science skill is 65 or higher, you may attempt to engage the turbines to flush the Legion troops out. This will require manual intervention. Go back to Power Plant 04. On the east side of Power Plant 04, go up the stairs in the middle of the room between the two large tanks. On the east wall there is an exit to Hoover Dam. There is a manual release valve along the river to your left between the broken retainer wall, just after the second sandbag wall. Return to Power Plant 03.

Proceed to Power Plant 02, then Power Plant 01. Climb the stairs on your left in Power Plant 01, then proceed north along the balcony. Follow the passage along the left, then go up the stairs, and exit via the elevator to the Hoover Dam Visitor's Center. At this point, you will begin to notice assistance appearing from companions and factions you have helped out along the way. Exit via the double doors on the east side to Hoover Dam.

Proceed to the north along the curved walkway above the dam. Take care along the entire dam not to go too quickly or you will end up with enemies flanking you. Enter the Hoover Dam Checkpoint, then exit on the other side. An NCR Ranger Veteran Commander will thank you for your support, then ask you how you would like them to coordinate their assistance. With a Speech skill of 85 or higher, you can use the option "You have superior firepower. You can overwhelm them if you push the advance on them." If you choose this, they will attack the Legion soldiers without your aid and give you a clear path to the Legate. The other options are "I need some snipers to cover my advance.", "Have your unit follow me, we'll assault them head on.", and "I can manage this without your assistance." Pick whichever suits your play style the best.

After crossing the dam, head left along the bridge to the east, and continue, following the road. Enter the Gate to The Legate's Camp. Head east into the camp, following the path up the side of the ridge. You will fight Legate. (With a speech skill of 100, you can pass a series of skill checks, and avoid fighting him.) After killing him you should notice the gate to the camp has closed. Head back to it. The gate will explode, you will meet with the General. Enjoy your endgame.

Bugs

 * The crazy nightkin that spawns near Brooks Tumbleweed Ranch and trys to sell you tumbleweeds may spawn along the top of Hoover Dam during the battle


 * Occasionally, leaving Power Plant 1 without completing the optional quest marker, you will randomly fail 'Don't Tread on the Bear!' and all NCR forces will go hostile to you. (Confirm, PS3, PC)


 * Sometimes the Legate will not register as dead and you cannot continue with the quest (PS3)


 * There is a bug where your followers ( if any are brought) will not follow you unless you tell them to


 * If you swim across, NCR will turn hostile.


 * If you have ED-E as a companion when you choose to activate the quest, for some reason he will not follow you into Power Plant 4, leaving him behind in the locked room.


 * Useing setstage 00136166 100 spawned none of the needed minions to continue the mission regardless of the fact that the player has progressed and done all available side missions nad fought through to the legates camp (as the previous mission had bugged) (PC)


 * If the final missions have bugged out while trying to help the NCR (as happens if you complete Wild Card: Finishing Bets before For The Republic Part 2), you can use setstage "00136166" 100. You then fight your way to the legate camp but Legate Lanius won't appear. You can then use setstage "00133075" 20 and General Lee will appear as you go to leave the legate camp, thereby starting the endgame sequence. I have tried spawning a Legate Lanius but he is immediately hostile and set to essential so he's not much use.


 * Doors may require key and cannot leave Visitor Center(PS3) Confirmed for Xbox 360 as well (I came here to find a solution.) SOLUTION: apparently if the staging area is left too soon after the president's departure, the doors will not unlock. They appear to unlock shortly before the "Quest Completed" finishes fading from the screen. a safe bet would be to save just after the president finishes the speech, and checking the doors before continuing onto another area.