The world has turned into a video game - or at least one particular city has.
One day out of the blue, an entire city is suddenly cut off from the rest of the world by way of an invisible force. You can see into the city's ongoings from afar and you can contact its citizens through all the usual methods but otherwise the city is completely isolated from the rest of the world. On top of that, the trapped citizens don't seem to be aware of their own predicament and after months of observation show no signs of resource shortages whatsoever and life seems to somehow be proceeding as normal for them.
It is only after year into this crisis that the world finally sees light shed on the matter. A mysterious group announces that they are the ones responsible for the event and that this was never meant to happen for so long. They call the phenomena a "Virtualization Bubble" that they have been working on to prepare for the future needs of humanity and that they had only intended to make a show of it for 3 months but rogue elements within their organization have stolen the keys to deactivating the system.
The rogues have escaped into the grid and are planning to take the system to its extreme potential and their actions could lead to extremely dire consequences for the world. To combat this they have sent out various core devices to agencies around to world to infiltrate the city by hijacking the consciousness of select individuals. The systems consume massive amounts of power and require clusters of living human brains to facilitate the hijacking so they had no choice but to share their secrets to prevent a catastrophe.
All is not as seems though as a member of the rogues approaches the primary hijacker candidates to enlist their help in secret. There is a much bigger game at hand and it is these primaries who have the key to turn things around for humanity.
People do die for good if anyone does die in the city so it's important their "player characters" are kept alive and in good condition and the rogues in the city and out in the real world have no way of contacting each other so it's all cloak and daggers for everyone.
Life inside the city still continues on as normal but there are various spots within it show clear signs of experimentation in the form of video game glitches. Interacting with the glitches result in unpredictable effects on the players. One of which happens to manifest on one of the primaries real self outside the city and can be utilized outside the bubble. The real twist is that the world has long been inside a big bubble too and that the city's isolation is the first step to overriding it but the group that discovered the secret of the world themselves have no idea what will happen if they do that.