This is season 4 episode 1 just in case you want to search for it specifically. When the episode first began I wasn't sure I had the correct TV show, because it seemed very different than other episodes as it resembled many episodes of Star Trek, which it is clearly trying to mimic.
I later found out that one of the writers for Black Mirror is a huge fan of Star Trek
What seems so odd about this episode from the start is that it has elements of comedy in it, which is something that I don't ever recall be a part of Black Mirror's normal operations. Nearly all other episodes are filled with serious drama and dread.
The story follows Robert Daly, who is the Chief Technical Officer at a company called Infinity, who distributes a wildly popular MMO-type game called Infinity. It doesn't seem to matter that he is a high-ranking officer in the company and also a co-founder, Daly receives very little respect by the people in his office. They are actually quite abusive towards him or simply ignore that he is there at all.
Robert Daly is played by Jesse Plemons and if that name doesn't ring a bell he was a later addition to the Breaking Bad series and is a terrific actor. I was enthusiastic he was involved in an episode.
A new programmer starts work at Infinity named Nanette Cole, and she is quite an admirer of Robert Daly's programming skill and is also a fan of his all time favorite TV show called Space Fleet.
She seems to genuinely like him and it almost appears to be flirting with him, until the other partner at Infinity, who is a more traditionally charming person comes in and ruins the interaction. You feel bad for Daly at this point.
Dejected and deflated, Robert returns home to his very routine life. As it turns out Daly has a modded build of the same MMO game at his home, but is somehow able to collect the DNA of whoever he wants from the real world and transplant them, including their consciousness , into his personal version of the game. In Daly's personal version of the game he is the captain, he is assertive, and everyone respects him. He is everything that he actually isn't in real life (does this sound like any keyboard warriors to you?)
He is also omnipotent in his version of the game, meaning he has complete control over everyone else on board. He is capable of turning other inhabitants into other creatures, force choking them, basically whatever he wants.
When Daly completes the "missions" that he can't possibly fail since he is omnipotent in the game, he will exit the game, leaving all of the others (who are replicas of his coworkers) trapped inside the game simply waiting for his return.
While it isn't explained if these "people" actually are the conciousness of their real-world counterparts, their lives inside the game are presented as real and basically they all simply live in a prison that never ends. They don't need to eat or drink, and they can't escape because Robert Daly controls every aspect of the world they live in. They respond to him based out of fear and now your emotions towards Daly change because as it turns out he has a rather awful dark side to him.
I've probably already revealed too much about the plot in this one, but it has the same spooky ambiance that the other episodes of Black Mirror have and this is a big part of the reason why it is in my top 5 favorite series of all time.
In case you have forgotten from past postings about Black Mirror, none of the episodes are tied to any of the other episodes, so by starting on season 4 (if this is your first one) you are not ruining any prior story-line.