While cleaning up a Hydra cell, the Scarlet Witch, a totally-not-mutant-you-heard-that-20th-Century-Fox? who can do pretty much whatever the script requires her to, puts some bad dreams in Tony Stark (a.k.a. Iron Man)'s head. So he and Bruce Banner (Hulk's non-green alter ego) decide to use the magic thingy out of Loki's staff to create an A.I., Ultron, that will defend the earth from alien attack, achieving, as Tony puts it, 'peace in our time.'
Now, had Steve Rodgers (Captain America) come into the room at that very moment, this would have been a much shorter film. Anyway, Tony's project has a similar effect to Neville Chamberland's surrender of the Sudetenland: aided by his sidekicks Scarlet Witch and her twin brother, Quicksilver, who, despite having the exact same name and ablilities, is totally not the same one from the X-Men, Ultron sets about bringing down his creator and destroying humanity, all the while evolving himself into his ultimate form. Only the Avengers can bring Ultron down, but that's not an easy task, as they're at odds and on the run (thanks to Hulk's uncontrolled smashing).
Now, I've seen people whinging on social media about how Age of Ultron wasn't as fun as the first Avengers movie, and yes, I would have to agree in a way. But what Ultron lacks in playfulness, it make up for in maturity, not unlike The Emperor Strikes Back or The Godfather Part II. Ultron is a lot darker, but also a lot deeper. In creating Ultron, Tony plays God not unlike another well-known fictional scientist, and, like Dr Frankenstein, his creation also turns on him. Meanwhile, Bruce's Jekyll-and-Hyde-like dualism is further explored, as, on the one hand, his relationship with Natasha (Black Widdow) makes him more human, but, on the other, he finds it increasingly difficult to come back from his Hulk state. Futhermore, Ultron rectifies the first film's under-use of Hawkeye in allowing us a greater look into Clint Barton's private life.
Age of Ultron provides some great cameo moments, bringing in characters from the Iron Man, Captain America, and Thor franchises, plus S.H.I.E.L.D. favourites like Nick Fury and Maria Hill. But where were Pepper and Jane? And it didn't seem like Cap and Thor were given very much to do (perhaps understandable, as they've got their own movies coming up). Anyway, there were some nice character moments showing just how much the Avengers have bonded as a team, including a great sequence when they all try to lift Thor's hammer, which is brilliantly funny with an even better payoff later in the film.
I just wish Marvel would realise that there are other ways to end a film other than destroying a city.