When Joss Whedon set out to make his first Avengers movie, the task seemed an unprecedented pop-cultural juggling act: Bring together heroes from four separate Marvel franchises with very different aesthetics—from World War II pulp to extraterrestrial Nordic polytheism—add in an overarching storyline about the spy organization SHIELD (along with a half-dozen affiliated characters), and try desperately not to fall flat on your face. Against all probability, Whedon utterly triumphed, producing an action mega-blockbuster with the soul of a drawing-room comedy.

For Avengers: Age of Ultron, that already ridiculous challenge has been upped exponentially. In addition to all the above, the movie throws into the mix four new major characters (Ultron, Scarlet Witch, Quicksilver, and the Vision); all of their respective origin stories; cameos of various sizes by fellow Marvel personalities War Machine, Falcon, Peggy Carter, Heimdall, and Erik Selvig; and the ever-heavier burden of serving as a bridge between all the Marvel movies that have been made to date and all those yet to come. (Hello, Black Panther’s Wakanda!) It’s the cinematic equivalent of juggling chainsaws while riding a unicycle. On a tightrope. Across the Grand Canyon.

To say that Whedon succeeds only in part scarcely counts as a criticism. (To belabor the metaphor: He may have dropped a chainsaw or two, but he got across safely.) Age of Ultron is a strong opener for the summer blockbuster season; it’s just not a mind-blowing quasi-revelation like its predecessor. The sharp, interpersonal dramedy that made the first movie such a delight is again present in flashes, but not infrequently it is drowned out by the noisy, inevitable need to Save the World.

The story begins where it left off. Not at the end of the previous Avengers, mind you—that would be too straightforward—but at the end of last year’s Captain America: The Winter Soldier. Having located the wooded alpine lair of Hydra commander Baron von Strucker (Thomas Kretschmann), the Avengers proceed to assault it. Therein they find both Loki’s scepter and a pair of decidedly unfriendly super-powered twins, Pietro and Wanda Maximoff, played by Aaron Taylor-Johnson and Elizabeth Olsen, respectively. (The super-monikers are never used but, yes, these are Quicksilver and the Scarlet Witch.)

Upon returning the scepter to the Avengers’ spiffy Manhattan HQ, Tony Stark (Robert Downey Jr.) and Bruce Banner (Mark Ruffalo) proceed to examine the gemstone embedded in it. This stone (Marvel fans will long since have deduced its identity) leads them to a breakthrough in artificial intelligence, enabling the creation of a Stark pet project, the “Ultron” program: robotic peacekeepers who can do the Avengers’ job for them when they are otherwise engaged. Not that Stark and Banner bother to tell anyone else about their little breakthrough. “I don’t want to hear the ‘man was not meant to meddle’ medley,” Stark explains. “Peace in our time—imagine that.”

Alas, peace never gets much beyond the imagination stage. No sooner does Ultron (voiced by James Spader) awake than he goes all Modern Prometheus on the gang, in the process breaking up a perfectly good house party, destroying Stark’s butler/operating system Jarvis (Paul Bettany), and announcing, with Spaderian relish, his intent to eradicate humanity. Oops.

From there the movie proceeds largely as one might expect. Ultron creates an army of metal Mini-Mes and recruits the Maximoff twins to his cause. Wanda, a telepath, messes with the Avengers’ minds—to particularly dramatic effect with Banner/Hulk—and Pietro runs circles around them, though never, alas, to the tune of Jim Croce’s “Time in a Bottle.” The Avengers crisscross the globe—East Africa, Oslo, Seoul—in pursuit of their new nemesis. Along the way, Captain America (Chris Evans) polices his teammates’ foul language, Black Widow (Scarlett Johansson) tries to make us forget that she had a much better haircut in Winter Soldier, Hawkeye (Jeremy Renner) reveals a Dark Secret, and Thor (Chris Hemsworth) flexes his steroidal-Shakespearean biceps. Nick Fury (Samuel L. Jackson) makes a dramatic entrance or two.

There are, of course, several action sequences in which the Avengers fight with Ultron, with the Maximoffs, and among themselves. These all culminate in a long final showdown back in Sokovia, the same fictional Eastern European nation where the movie began at Strucker’s lair. I confess that it’s here that Age of Ultron began to lose me. The battle with an army of innumerable CGI opponents felt altogether too familiar, even if this time Whedon exchanged aliens for robots. Moreover, one of the advantages Marvel has always held over DC Comics is that it’s more typically set in the real world (e.g., New York City over Metropolis), and try as I might I found it hard to get terribly worked up about the fate of Sokovia. And while I genuinely appreciate Joss Whedon’s sentiments on reducing collateral civilian damage, an awful lot of the final act revolves around the rescue of nameless, line-less, interchangeable Sokovians. Give me a skeptical NYPD sergeant won over by Cap's heroics any day.

There’s plenty to like in Age of Ultron. The dialogue is sharp and the actors mostly hit their marks—Taylor-Johnson and Olsen’s tongue-y accents notwithstanding. A series of gags about Thor’s hammer is a particular pleasure, and there’s an unexpected soulfulness to Bettany’s rebirth as the Vision. But the hectic demands of so many character introductions and plot twists and battle sequences and Marvel tie-ins inevitably crowd such moments to the margins. A playful romance between Black Widow and Banner never quite catches fire, and Hawkeye’s humanizing backstory feels half-hearted. There’s simply too much going on: Age of Ultron covers far more ground in two and a half hours than Marvel’s recent Netflix series, Daredevil, did in thirteen.

After shooting the first Avengers, Whedon said his “detox” was the unassuming black-and-white adaptation of Much Ado About Nothing that he shot at his own house during a two-week “vacation” from post-production. By the time it finally winds into its conclusion, Age of Ultron feels as if it’s in need of a detox of its own, or at least a mild purge of some sort. It’s telling that the single loosest, most entertaining sequence in the entire film is the wisecracking party at Avengers HQ before Ultron even shows up. Which raises the heretical thoughts: What if the villain had never shown up? What if the world didn’t need saving at all? What if the movie had dared to offer less “ado” and more “nothing”?