The game proceeds according to the player's decisions and involves many different events, factions, and characters, but the main storyline follows the player's pursuit of Benny to both settle the score and retrieve the platinum chip. Eventually, after finding Benny and the chip, the Courier finds themself in the middle of a conflict between three factions: Caesar's Legion, a group of Roman-esque slavers, the New Californian Republic (NCR), an expansionist militia government, and Mr. House, the enigmatic de facto ruler of New Vegas in command of an army of Securitron robots. Each of the three sides aim to control Hoover Dam, which is still operational and supplying the South West with power and clean, non-irradiated water, and thus control of the dam means effective control of the region. It is revealed that Mr. House, a human from before the Great War andsurviving via a contained life support chamber, ordered the platinum chip's delivery before the war. The chip is adata storage device with a program that can upgrade the Securitrons to a greater level of combat effectiveness, and was stolen by Benny as part of a scheme to take over House's security and claim New Vegas for himself with the help of a reprogrammed Securitron, Yes Man.
The player has the option to pursue one of four paths - fighting for Caesar, NCR, Mr. House, or taking up Benny's plans to take New Vegas for their own with Yes Man's assistance. After a line of quests where the player deals with outsider factions to determine their role in a looming battle, the player is notified that Caesar's Legion is attacking Hoover Dam and they must take part to decide the outcome. As the Legion strikes the Dam, led by the fearsome Legate Lanius (who becomes leader of Caesar's Legion if the player allowed Caesar to die from a brain tumor prior to the battle), the NCR protects its position under General Lee Oliver. Depending on the faction sided with up to the battle the player will either conquer the Dam for Caesar, defend it for the NCR, or connect the Dam's systems to House's network so he or Yes Man can take control. The game concludes with a slideshow showing the results of the player's actions, the battle for Hoover Dam deciding the faction that comes to power over New Vegas and the Mojave, and the fates of the various other factions based on how the player negotiated with them and which of the major factions emerged dominant.
