Well, in short:
Its a soon future, with really advanced high-tech technology, something like now:
Governements have almost collapsed, they're just the shadow of themselves, it has somewhat be remplaced by corporations, big business like Apple, Microsoft, Coca Cola, etc... who has as much money as some state (becoming some kind of "private states" in themself), you have Arasaka, some kind of evil google wanting to rule over the world (not something cliché like "we're the evil bad guys" but just a total "capitalist-imperialsm" stain of mind), Militech, and american weapon maker, Petrochem (oil/chemicals), etc....
And the point in cyberpunk, is that there are no "plot", well, in itself there is nothing, no "big evil satan to kill to save the world".
But, still we can assume some few plot line.
In 2022, there were a chaotic world wide corporation war between Arasaka and Militech (they can't stand each other), who really degenerated in violence, experimental weapon, etc... Arasaka ended having their ass kicked by a bunch of punk, and I'd say they were somewhat punished for the mess they created (like the german after WW1).
You can figure that, knowing how Arasaka have an "Hardcore winner" mind, they won't buy this.
I can see the main plotline being to prevent Arasaka from unleash the dog, can be a "Soulkiller 3.0" (a program who burn your brain), some advanced Mech or whatever to take control of Night City, and you'd have (well, it's cyberpunk , so you'd be forced to do it in a way or another) to prevent Arasaka to become as powerfull as they were.
But a Cyberpunk game has no real plot, it's just like real life, there are plenty things to do, no world to save, it's just you trying to end the day alive.
That said, you'll have to go from being a drug dealer, a body guard to being a greedy corporate to a talented hacker.
You can watch movies like Strange Days or the last Judge Dredd movie "Dredd" to have an idea of how the game is gonna be, in a cyberpunk world, it's totally different from what you are used to play, in this world you have no "power" over what surounds you, you just have to deal with it.
You do your shits and try to avoid being thrashed by a gang of homeless peoples.