You step out of Doc Mitchell's house and Goodsprings has fallen: the townsfolk are Ganados.
Resident Evil 4 is really in it. Every time New Vegas starts, Goodsprings Evil reads Resident Evil 4's own sounds from your installed RE4 (2005) copy. The Ganados shout their real RE4 lines ("¡Allí está!", "¡Un forastero!", "¡Te voy a matar!", "¡Te voy a hacer picadillo!"), and the Merchant greets you with his "Welcome!".
What you get
Goodsprings falls. Ten Ganados (Goodsprings' own settlers turned, with machetes, shovels, hatchets and cleavers) swarm you as you reach town. The real townsfolk are gone until you clear it.
Headshot stagger and Leon's kick. Shoot a Ganado in the head and it staggers. Press Activate next to it to kick it, and every Ganado around it, flying.
Las Plagas. A Ganado killed by a headshot can lose its head, and a Plaga may burst out of the body.
The Merchant. A hooded stranger by the Prospector Saloon sells three exclusive guns, each firing with its RE4 gunshot: the Matilda, the Striker and the Broken Butterfly. He also has ammo and stimpaks. Ganados drop caps.
Clear the last Ganado and Goodsprings' people return.
How to play
Start a new game, or load one, and head into Goodsprings. The outbreak begins the moment you step out of Doc Mitchell's house, or the next time you arrive in town. Single player.
You need
Fallout: New Vegas (Melty installs xNVSE for you).
Resident Evil 4 (2005) installed from Steam (the Ultimate HD Edition). Without it the mod still runs, but the Ganados and the Merchant are silent, and the game tells you so.
No game files are included: both games are read from your own installs.