I have no idea what Fallout is all about. I know it's a game but haven't a clue. So i will hit this completely blind - as much as i did with The Last of Us
Potted description:
History diverges from ours during the mid-20th century, no specifics but essentially development follows down the sort of path that 1950s scifi 'predicted', with nuclear power being far more widespread (small nuclear reactors powering cars, battlesuits, floating robots etc). The 1950s 'Americana' aesthetic thus dictates a lot of the culture & architecture of the "future". Throughout the 21st century increasing wars are fought over dwindling resources, primarily between the US and China, which culminates in the 'great war' of 2077, a 1-day Nuclear Holocaust that pretty much wipes out global civilisation. The games are generally set 100-200 years past this as the series goes on, and take place in various locations across the US, with a variety of regional factions in play.
The Vaults were a series of underground bunkers made by Vault-Tec in the face of the possible upcoming disaster. On the face of it these were purely protective for anyone buying space for their family, however the real truth was that each Vault ran its own horrific social engineering experiment, carrying out research on the unsuspecting human population once sealed in.
The games aren't really linked and there are very few constants that turn up (outside of the guns, robots, generic post-apoc raiders & mutants etc). One is the Brotherhood of Steel, a national faction descended from the US Army who extensively use the powered battle suits and believe nobody else should have access to advanced technology. It's their airship seen in the trailer, and they can be considered either allies or villains in the different games depending on which local factions are sided with/quest lines followed.
Edit: I should also say that I don't expect the series to follow the plot of any particular game too closely, several of them have followed a Vault-dweller leaving the vault for the first time and having to learn how to deal wth the wasteland, but the overall stories are different each time. We do see the flag of the NCR (New California Republic, a relatively large and civilised area of the West Coast with a functioning Govt & Military) in a battle with the Brotherhood, and one of the main characters seems to be one of the Brothers by themselves, so maybe an Outcast. So on the surface the Brotherhood could be the villains of the piece. But it could all be a smokescreen!
