Instructions
Avoid being reckless and violent inside the dam unless the base is under attack. If the NCR looks favorably upon your character, you may be asked to visit the dam for certain mission instructions. Assaulting guards or stealing from the base can evoke a violent reaction from the military personnel on site, making it very difficult to leave the dam alive.
Crouch and use stealth to remain inconspicuous around NCR guards if you have soured your reputation with that faction. NCR guards patrol the entire dam, particularly on the side nearest New Vegas. If you aggravate these guards, the entire base will relentlessly pursue and attack you, making it extremely difficult to leave the dam. If you are sneaky, however, you can loot the area and exit without much trouble from the NCR patrols.
Complete all the tasks for whichever final mission you have selected, based on your alliances. For example, if you are allied with the NCR, your final mission involves holding the dam and defending it from a major invasion by Caesar's Legion. A prompt warns you before the final mission starts that you will not be able to return to the world map after the mission begins. You can leave the dam, however, by fighting your way to the other side and completing any necessary mission objectives along the way.