Lore in Fallout 76, in the Fallout universe has a long history but compared to other fallout games Fallout 76 would be set earliest in the timeline.
The overall story arc of Fallout 76 follow the exploration of the first Vault Dwellers to leave the first bunker (Vault 76) opened, 25 years after the bombs that obliterated the U.S of A hit. The adventure begins on Reclamation Day, a holiday to celebrate being the first people to traverse the wasteland and reclaim America, or more specifically Appalachia.
- This explains the lack of human NPCs in the world.
You meet a Mr. Handy robot shortly after leaving the vault and it sets you on your path to learn about the Overseer of Vault 76; she was the leader of your vault who left the vault a few hours prior to everybody else on 23 October, 2102. She left to secure the missile silos that you eventually have to do for her, on the orders of Vault-Tec, and leaves a trail of Holotapes and Journals for you to follow, to help her accomplish the mission. This is what the main quest line revolves around; following her clues to ultimately takeover one of Site Alpha, Site Bravo, and/or Site Charlie.
- Initially supposed to oversee Vault 101, however her engagement to a man called Evan prevented this, and she was given the options to either leave her homeland of Appalachia or become the Vault 76 Overseer without Evan. She chose the latter, and finds him as a Scorched during her travels after.
She directly puts you on path for First Contact, and the side quest Personal Matters. However, the Holotapes are littered across most of the main quests and shape the narrative. The Overseer's Log - Top of the World guides you to Top of the World, the center of Appalachia.
The 'Responders' are a faction of people from different emergency services, that formed shortly after the Great War. They were assembled to continue to help the last of the humans, as they did in their occupations prior to the hellfire. The remains of their work can be found all across Appalachia in emergency camps and buildings, with their heart logo plastered at the front.
The quests from Thirst Things Thirst through Into the Fire, are all invoked in some form by the Responders. Completing Second Helpings, grants you the honorary status of 'Full Volunteer', which gives you full access to the Responders Terminal, including contacting the Main Headquarters. Into the Fire is one of a few examination quests, that require you to pass a set of tests in order to unlock a rank in a particular faction, in this case it's the fire department sector of the Responders; it's the first, but it definitely wont be the last.
Top of the World
The The Missing Link quest introduces you to Signal Strength, Flavors of Mayhem, Key to the Past, and requires you to complete them before being able to proceed with it. Here you'll be introduced to Rose, a sociopath of a robot that has a lot of demands to help you acquire what you need -some of which are more absurd than others.
The demands include tasks such as gathering parts, locating keys, and killing a karma-juiced up bear. Once you help her, she grants you access to a set of quests which contain more of the same type of objectives. You're able to collect the Broken Uplink by the end of The Missing Link which will then require you to head over to Abbie's Bunker.
Vault Dweller, This is your Tape.
In Abbie's Bunker, Early Warnings then Reassembly Required will have you locate the components for and then creating Upgraded Motors. You'll then need to set them up along a road of Scorched Detectors then head back to the Top of the World to return Rose's Holotape that you acquired during the former quest. This will trigger the Coming To Fruition quest that requires you to collect a Master Holotape which you have to plug into a few terminals before uploading it to a relay tower.
Another of the factions in Fallout 76, the Brotherhood of Steel is the next of the ones you have to join to progress further in the main questline. The Brotherhood is a band of survivors with a military or special forces background that have merged into a militia-like organization, that aims to protect the remaining humans.
After you complete Coming To Fruition, you'll be set on course to completing a series of quests for the Brotherhood of Steel, starting with Defiance Has Fallen. In this mission you learn through the Letter To Tommy of the location of Fort Defiance, the homebase of operations for the Brotherhood of Steel in Appalachia. The Back to Basic quest will allow you to achieve the requirements in terms of training, while Recruitment Blues does so with the mundane paperwork.
While those quests provide complete entry into the Brotherhood, Belly of the Beast requires completion in order to finish off the Brotherhood of Steel's quests; this one will need you to track down the remains of the Appalachian leader of the Brotherhood, Lieutenant Elizabeth Taggerdy for her ID to gain access to her Terminal in Fort Defiance.
Established in secret before the events of the Great War, the Enclave was a shadow government that became the self-appointed authority after the collapse of the country. The headquarters at The Whitespring Resort run by MODUS, an AI with a bit of a dodgy personality. The Uncle Sam quest will be the first of the Enclave's, and will involve finding Sam Blackwell's Bunker. The Sam Blackwell Interview Notes at the Charleston Herald will give you a location.
Once inside, Bunker Buster will commence and you'll learn of the existence of the Bypass Holotape. The only problem is that all that you know is that it is hidden in a Deathclaw Nest, in this location. Once you find it, you gain access to the deeper areas of the bunker and eventually find a whole set of documents that help plug in the holes in the story of the Enclave, and give you further access.
At The Whitespring Bunker, you'll meet MODUS and have to do more administrative work for him, for the One Of Us quest. By the end, Officer On Deck will provide you with the rank to continue on the ultimate quest to reach and access the Launch Console.
While still technically part of the Enclave's plans, the subsequent I Am Become Death involves gathering the final requirements to use the nukes, and activating them. Mission: Countdown gives a detailed plan as to how to approach whichever Silo you choose to, while Rundown provides a complete play-by-play of the obstacles that you'll encounter. Death From Above is a simple run-through from the time the location of the launch is selected, to the launch itself. If the Prime Fissure is targeted, Scorched Earth will commence, which will have you deal with the Scorchedbeast Queen.