There are Fallout mods, and then there's Fallout: New California.
Fallout: New California, aka project Brazil, is a mod over five years in the making. It totally converts Obsidian's Fallout: New Vegas and launches for free on PC on 23rd October - potentially around the same time Bethesda's recently-announced Fallout 76 comes out.
Fallout: New California tells a prequel to the wonderful Fallout: New Vegas. It's set in the year 2260 and begins in Vault 18, which is set in post-apocalyptic Los Angeles. As you'd expect, you make your way to the surface where you find a number of factions fighting for control over the California wasteland.
The video, below, shows the lengths the developers have gone to to make Fallout: New California as robust an experience as possible, with full voice acting for characters and entirely new locations (currently, there are 16,000 lines of dialogue). The developers have finished the mod's main quest, but there's still work to be done to add side quests.
Fallout: New California comes out at an interesting time for the Fallout franchise. With Fallout 76 reportedly incorporating online elements into the series, a traditional single-player experience - albeit one fan-made - could sate the appetite of some fans who don't fancy playing with others.