(CNN)The powers of Marvel's all-star superheroes go a bit wobbly in "Avengers: Age of Ultron." Faced with the daunting prospect of topping the surprise and excitement of 2012's The Avengers, the third highest-grossing film of all time, writer-director Joss Whedon mixes some brooding down-time in with the abundant spectacle. To be sure, series junkies will get their fix from the sheer massiveness of the exploits, but at least two of the big action scenes are lackluster, while the climax and resolution could have been worked out in more complex, less rote ways, so as to further increase intrigue and anticipation for "Avengers: Infinity War Parts 1 and 2," already scheduled for release in May of 2018 and 2019, respectively. Not that any of this will matter much, since the pent-up excitement among the enormous international fan base is so intense that nothing will keep the summer's presumed biggest franchise blockbuster from soaring to and beyond the one-billion dollar threshold internationally. Hands-on producer Kevin Feige and his associates have built a cinematic empire quite unprecedented in Hollywood history, a veritable solar system of staggeringly profitable individual franchises unified by the overpowering collective force of the Avengers. So while sideline enterprises like the new "Daredevil" TV series continue to pop up, the company can be so confident in the enduring appeal of its theatrical mainstays that it recently published a release schedule for its remaining big-gun commercial titles from now through the end of the decade. At this point, no one would be willing to bet on when a sense of terminal deja vu might set in to bring it all to an end, and Comic-Con can plan its main events years ahead. In the meantime, the key points of interest surround how many surprises and twists can be wrung from a format and set of expectations that demand great fealty from core fans; any significant deviations are taken as personal betrayals by the hardest-core geeks. Last summer, "Guardians of the Galaxy" showed that Marvel could play it a bit more fast and loose than it generally does, but the big-name franchises still seem sacrosanct. And so it is with "Avengers: Age of Ultron," which at moments takes a peek down some shadowy side roads but ends up mostly zooming along the main highway to deliver what the audience wants rather than something even a little bit different. Picking up where last year's "Captain America: Winter Soldier" left off, the new film, without preamble, dives right in to show the Avengers dispatching the remnants of the nasty HYDRA organization in a hectically and indifferently staged forest combat scene that leaves Clint Barton/Hawkeye (Jeremy Renner) worse for wear while also introducing two new adversaries, twins Pietro and Wanda (Aaron-Taylor Johnson and Elizabeth Olsen). Victory allows for some passably amusing scenes of the heroes blowing off steam: The favorite party trick of Thor (Chris Hemsworth) is to challenge all comers to lift up his hammer; Natasha/Black Widow (Scarlett Johansson) and Bruce Banner/The Hulk (Mark Ruffalo) pursue a little mating dance in which her amorous interest is predicated upon his retaining his human rather than superhero form; and Tony Stark/Iron Man (Robert Downey Jr.) and Steve Rogers/Captain America (Chris Evans) take stock of the advances their old Nazi nemesis Strucker (Thomas Kretschmann) appears to be making in the artificial intelligence department, specifically with the most sophisticated humanoid ever devised, Ultron (voiced with marvelous robotic nuance by James Spader). What's behind Warner Bros.' risky move to release nine movies . Ultron is a cool and sophisticated creation; what he lacks, of course, is a heart, which is what makes him such an imposing villain. A sleekly designed robot you might even call handsome, he makes an excellent intellectual and smart-ass sparring partner for Stark, but when he first appears, he's still on training wheels. However, he does recruit Pietro and Wanda to his cause, an easy matter since Stark killed their parents. He's not yet entirely ready to conquer the universe but, in their first skirmish with him, the Avengers are sufficiently outclassed to begin worrying. Could 'Star Wars' open at $500 million? Licking their wounds at the "safe house" of Hawkeye's farm, the Avengers go into a funk. The impatient Thor quickly takes off "to find answers," Bruce resists Natasha's desire to get something physical cooking between them; Stark, lamenting that "Ultron is tryng to tear us apart," consults with old cohort Nick Fury (Samuel L. Jackson, briefly), while Mad Men's Linda Cardellini, playing Hawkeye's stand-by-your-man wife, is stuck with the (intentionally?) funniest line of dialogue in the film: "You know I totally support your avenging." While some of his partners wallow in disarray, Captain America heads for Seoul, where the next evolutionary step is to emerge via a device called "the cradle," which will hatch mass-produced android soldiers that will pave the way for Ultron's world domination. But a major chase through the city involving a runaway subway train falls flat due to basic conceptual silliness and poor action continuity. Of course, the Avengers ultimately get it together to do the kind of butt-kicking they're supposed to do, and a very welcome addition to the team comes in the form of the android called Vision. Red-faced and green-garbed, Vision is given a striking profile and overall presence by Paul Bettany (heretofore limited in the Marvel world to vocal work as Stark's computerized butler J.A.R.V.I.S.), and it can be hoped, if not assumed, that this most intriguing character will play an even more important role in the final two Avengers installments. 'Galaxy Quest' TV series in the works . Ultimately, Whedon's efforts to invest the heroes with a degree of uncertainty and vulnerability comes off as half-baked, as such an effort can only go so far due to the nature of the material. After all, these are comic book characters defined by their double identities; a third dimension is neither required nor perhaps even desired. If ending on a dramatic cliffhanger note had been desired, the elements were there for the taking; including a semi-tragic component along with mystery about Ultron's ultimate fate would arguably have only further cranked up anticipation for the coming chapters. But, then, what does that matter when the automatic attendance of millions is assured? "Avengers: Age of Ultron" succeeds in the top priority of creating a worthy opponent for its superheroes and giving the latter a few new things to do, but this time the action scenes don't always measure up and some of the characters are left in a kind of dramatic no-man's-land. The returning series actors acquit themselves in the expected agreeable manner, while series newcomer Andy Serkis has a terrific couple of minutes as a tough but stressed South African criminal. ©2015 The Hollywood Reporter. All rights reserved.
'Avengers: Age of Ultron' hits theaters May 1 . Critic: Movie doesn't quite measure up to the original from 2012 .