Spider-Man: Far From Home had a lot to keep quiet ahead of its release. The entire plot, for instance, revolves around the fallout from the events in Avengers: Endgame, as Peter Parker steps into Tony Stark's shoes for the first time and saves the world. Moreover, the villain of the piece, Mysterio, was also a massive spoiler, as everything leading into the film suggested he was a friend. So who is Mysterio in Spider-Man: Far From Home? The answer lies within the Iron Man films. Warning: Spoilers for Spider-Man: Far From Home follow.
When Spider-Man: Far From Home opens, Nick Fury and Maria Hill have just shown up to a small town in Mexico, where a tornado "with a face" had just destroyed the main drag. Just as Fury and Hill are discussing if this is even a real mission for S.H.I.E.L.D., another creature, this one made of stone, rises from the Earth. Along with it, the strange Mysterio appears and takes it down.
According to this new hero, he has traveled to Earth via a multiverse wormhole from his own Earth, which was destroyed by these "Elementals," creatures, made of Earth, Wind, Fire, and Water. Earth and Wind struck in Mexico. Water hits in Venice where Peter Parker is on his class trip and gets roped into helping Mysterio. Fire is being tracked to Prague, where Nick Fury reroutes Parker's class trip to head to next, so he can assist a second time.
But this is all an elaborate ruse. There is no Mysterio, no wormhole, no Alternate Earth or multiverse. Mysterio is actually Quentin Beck, a former assistant to Tony Stark, who invented the technology seen in Captain America: Civil War, which Stark called "Barely Augmented Retrofitting." (See the scene above.)
Beck has taken this augmented reality he created and turned it into projections that make it look like the Earth is being attacked. He's in league with several embittered ex-Stark employees, including the man who was once in charge of Stark weaponry, who was famously dressed down in the original Iron Man for not being able to reproduce the Iron Man suit for Obadiah.
These ex-Stark men are here to create the world a superstar hero out of whole cloth. Electrical impulses make the readings seem real to S.H.I.E.L.D. sensors. The drones act as both projectors and weaponry to inflict damage, so the "Elementals" look like they are destroying cities. And Mysterio appears in time to save the day in each instance, convincing the world to love him.
That they attacked where Peter Parker was on vacation was no accident either. Beck plans convince Parker to trust him enough to hand over the glasses Tony bequeathed him, which via EDITH, control the entire Stark industries mainframe. These ex-Stark employees aren't just bitter that "heroes in capes" are taking credit for their work, but that in the end, Stark left his work in the hands of a teenager.
But when Parker catches on to Beck's plan, the savvy showman has one more trick up his sleeve. The drones record as well as project, giving Mysterio the footage to make it seem like Spider-Man murdered Earth's beloved new hero, and reveals Parker's identity to the world. That will be very interesting come Phase IV...