The Marvel Cinematic Universe whirrs along with Avengers: Age Of Ultron, which is a true sequel to 2012's The Avengers but also a reward of sorts for making it through the four smaller superhero flicks released in the interim. This is the social contract comic book fans have signed now, as Marvel Studios ends Phase 2 of its grand experiment. Put up with a steady stream of what we should start calling Marvelettes, each in itself the size of a traditional studio tentpole, and every few years you'll reap an even grander, star-studded megalodon of heroism so awesome it threatens the end of the world.

But just because there's a grand plan at work doesn't mean it's working. Age Of Ultron feels like it should be firing on all cylinders. It reunites the colorful cabal of Iron Man, Captain America, Thor, Hulk, Black Widow, and Hawkeye with their one true leader, beloved sci-fi writer-director Joss Whedon. Over 140 minutes, it allows ample downtime for quips galore: There's the bit where everyone takes turns trying to lift Thor's Nordic-god hammer, and the running gag where Cap, who was flash-frozen in 1942, inadvertently reveals his more prudish sensibilities. It introduces at least four new faces from the comics, with cameos from the film franchise's vaults. And it deals in transparent themes of evolution: the forward progress of humanity, of the evil it creates, and, ultimately, of the sort of heroism that must stop it.

And yet, the film lacks the giddiness of Guardians Of The Galaxy, the stylistic touches of Captain America: The Winter Soldier, the (relatively) deep introspection of Iron Man 3. It is, in short, more disposable than the mere mortal blockbusters it was designed to outpace. This should defy Marvel Studios mathematics. How can a movie with more stuff than its predecessors be less fun?

What was so engaging about the first Avengers movie, much more than the world-saving, was the thrill of seeing all these distinct personalities enter one another's orbits for the first time. They could have killed each other; instead, they figured out how to work together, Dirty Dozen-style. But all that tension is a nonissue for the sequel, which begins with six conflict-free good guys conducting a harmonious, impeccably choreographed raid on a Hydra stronghold. Iron Man (Robert Downey Jr. in helmet-vision) shoots death rays at the advancing hordes as Hawkeye (Jeremy Renner) picks them off from afar with his bow. Thor (Chris Hemsworth in full hammy pomposity) and Cap (Chris Evans) pull a combination hammer-shield maneuver that brings the house down. Even Hulk (Mark Ruffalo when un-greened) seems in control of himself. Later, Black Widow (Scarlett Johanssson, still clad in impractical cleavage-baring leather) will sooth him back to normal, with hints of a superromance in the air.

This is the purest state of the Avengers, the status the first film took its entire length to build toward, and it doesn't look much different from an Expendables or Fast & Furious movie. As fun as the action is, it drives home the inescapable point that this dynamic team must remain composed of static, increasingly stale characters. Whedon might be thematically pressuring them to evolve, but they can't, for fear of jeopardizing the never-ending Marvel movie mission. So instead, they must push and pull as a single unit, crowding the screen, with every Thor one-liner and every assurance that Hawkeye has value perfectly calibrated for group effect. Someone, anyone, please drive the Avengers apart.

You there, the snarky sentient virus with the body of a rejected Iron Man prototype and the voice of James Spader. What's your name? Ultron? You'll do for now. Born out of a failed attempt by Tony Stark/Iron Man to create an automated peacekeeping robot by cross-breeding his own computer program with an alien element, Ultron instead makes the philosophical leap to conclude that true world peace will lie only in the Avengers' destruction. He lives in the cloud and is at his creepiest when inhabiting a half-formed metal shell that can stumble and gesture like a zombie.

Ultron does briefly succeed in sowing doubt among our heroes, but only with a little help from the mind-control powers of genetically modified Eastern European refugee Wanda Maximoff (Elizabeth Olsen, flipping her brainwashed/brainwasher dynamic from Martha Marcy May Marlene). She tricks several Avengers into seeing visions of their greatest fears, leading to the film's best — and most Whedonesque — sequences: Cap's flashbacks to WWII pastels, Black Widow's icy KGB training. It also sets up a great battle between Iron Man and a possessed Hulk, who flails like a slippery, muscle-clad eel.

But we never really sense that a wedge is being driven between any of the Avengers. Instead of taking on the conflict, they just absorb it, turning Wanda (aka Scarlet Witch) to the light midway through and adding two more good guys in the process. Their ranks are so crowded, they'll soon be able to fill a Comic-Con by themselves. Why care about any one of them anymore?

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