After completing You'll Know It When It Happens, at the very moment you choose to side with the NCR for the final battle (thus completing For the Republic, Part 2, you'll be automatically taken from Col. Moore's office to General Oliver's Compound, where the General will begin briefing you. The player is given the chance to wait, as this begins the end quest for the game. If the player chooses to continue, Legion members attack the Hoover Dam and you fight your way through the power plant rooms and head towards their base east. 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) To complete the optional part of the quest, follow the staircase immediately to the left leading back upstairs to enter Power Plant 03. The console is on the second floor of Power Plant 03, on the southern wall. Passing a Science check (65) allows the player to use the terminal to engage the turbines and flush the Legion troops out. Completing this will require manual intervention; return to Power Plant 04 by the closest door. On the east wall, on your left as you re-enter the area, there is an exit to the Dam exterior. The manual release valve you need is to complete this part of the quest 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.