Sneaky superhero movie! It was another caped-crusader tradition that gave us the idea that heroes who don’t die in the line of duty live to become villains, but it took The Avengers to let it play out onscreen. The bad guys in Age of Ultron? Tony Stark and Bruce Banner. For real.

In Tony’s case, his villainy is externalized in Ultron, an AI creation that he has been working on for years but which he can finally bring to fruition now that the Avengers have reacquired Loki’s wondrous scepter from what’s left of Hydra (this is the opening sequence of the film); something to do with the extra computing power the scepter offers. Ultron (the voice of James Spader: Secretary, Supernova) has some of Tony’s (Robert Downey Jr.: The Judge, Chef) attitudes, but a glitch in its “birth” makes it go a bit cyber-insane, and it extrapolates Tony’s notion of world peace to mean “a planet without humans.” And now it’s gotten loose and must be stopped, natch. (It’s sorta nice, given Hollywood’s tradition of AIs always being fundamentally evil, that here, there’s at least a nod to the idea that something had to have gone wrong for an AI to turn evil. And there’s a further vindication of AIs as not something to be necessarily feared later on.) But it’s not so much the twisting of Tony’s attitude that’s the near-villainy here but that Tony was keeping yet more secrets — and Ultron is a huge one — from the people who are supposed to be his partners in saving the planet. The rest of the Avengers (sans Bruce, who reluctantly helped Tony jumpstart Ultron) learn about this new danger to the planet after it has tried to kill them all. This literally ruins the Avengers’ party.

In poor Bruce’s case, though, it really is he who has become a menace. Well, the Hulk has, anyway. In the middle of a battle rage that gets even more out of control than usual, the Hulk goes on a rampage that causes massive destruction in downtown Johannesburg, one that is stopped only when Iron Man — in Hulk-scaled power armor — steps in… and it’s not an easy thing, either. It’s a fairly horrifying sequence, watching friend forced to battle friend and knowing that Bruce doesn’t want to be doing this (never mind what the innocent injured people of Jo’burg must be feeling). His horror once he has transformed back into his human self, which continues through the rest of the film, is palpable — Mark Ruffalo (Foxcatcher, Begin Again) is fantastic — but also tough for us not to share in. The Hulk is dangerous to everyone, not just bad guys.

Our Heroes Tony and Bruce have finally gone full mad-scientist. It’s a fascinating upending of genre tradition that might make this flick work for nonfans, even though it assumes a lot of familiarity with what’s come before in the series, such as that you’ll know what Loki’s scepter and Hydra are about without having to be told. It’s not all gloom! Director Joss Whedon (Much Ado About Nothing, Dr. Horrible’s Sing-Along Blog) insures that Ultron continues the Avengers tradition of big, bold action blockbusters that don’t need to toss away thoughtfulness to remain pure popcorn fun. (I love the new antagonists — it’d be too much of a stretch to call them villains — in the superpowered twins, products of Hydra experimentation, Pietro [Aaron Taylor-Johnson: Godzilla, Kick-Ass 2] and Wanda Maximoff [Elizabeth Olsen: In Secret, Very Good Girls], whom readers of the comics will know as Quicksilver and Scarlet Witch. They have a big grudge against Tony that, in fact, underscores his not-implausible villainy.) Tony and Bruce’s second-guessing of their work is more comic-book soap opera than serious drama anyway, as are some of the other things we learn about the personal lives of the Avengers here: romance is budding between Natasha, aka Black Widow (Scarlett Johansson: Lucy, Captain America: The Winter Soldier), and Bruce, though she has to push it a bit; and Clint, aka Hawkeye (Jeremy Renner: Kill the Messenger, American Hustle), turns out to have a surprising secret. And the finale is truly sci-fi superhero action movie-movie whackadoodle, taking urban destruction to a new (antigravity-assisted) level.

The film isn’t without problems. Urban destruction — Seoul also takes a beating here — is getting old; I was a little disappointed to see Whedon evoke 9/11 imagery in the Jo’burg sequence, when he had previously avoided it in his first Avengers flick. The FX in the opening attack-on-Hydra sequence are surprisingly cartoonish. And 45-minute battle finales have to go: it’s overkill, and we’ve had enough. Can we find something new for these movies to do?

The cool thing is, Age of Ultron ends by suggesting that yes, maybe we can find something new. Big changes are initiated here that indicate that the team, at least, is going to be very different next time out. This is a franchise that isn’t afraid to move on from aspects that aren’t working anymore, and here any tiredness in some of the characters is confronted directly and dealt with. When I say that this is the weakest of the Avengers movies so far, I don’t mean that it isn’t still hugely enjoyable. And any complaints are mollified by the hint that this huge ongoing story isn’t going to shy away from refreshing itself as needed.