The weight of responsibility hangs over every character in Marvel’s “The Avengers: Age of Ultron.” It is a weight no doubt felt by writer/director Joss Whedon, who not only shepherded 2012’s “The Avengers” to its place as the third-biggest movie ever, but who also looked over and touched up scripts from every movie of Phase Two of the Marvel shared-universe films.

His last film for the Marvel-Disney empire opens this weekend, and “The Avengers: Age of Ultron” now finds Whedon in the ironic position of having to accommodate future installments by introducing characters and plot points. An overstuffed plot and a heavy concentration on action contribute to a breakneck pace and general lack of surprise in the movie, which mirrors the structure of the first film.

That isn’t to say that the movie isn’t at times wildly entertaining, maintaining that touch of sparkling wit Whedon is known for, even in the darkest hour. Another overriding theme is man vs. himself, as Whedon forces his heroes to reckon with their own hubris, fueling some very poor choices that result in something a whole lot more threatening than the neo-Nazi terrorist organization Hydra.

Ever since going through a wormhole in space to sacrifice himself in the last “Avengers,” Tony Stark (Robert Downey Jr.) has been tormented by nightmares. He wants to eliminate the need for a superhero supergroup by creating a shield for the planet that would defend against alien invaders and create “peace in our time” — a sort of advanced version of Reagan’s “Star Wars” defense umbrella perhaps (which always seemed better suited to a comic book anyway).

Through Stark’s meddling, and with the reluctant help of Bruce Banner (Mark Ruffalo), a sentient being named Ultron is created, and first visualized in floating, color-coded CGI blobs that look like the final showdown between Eric Bana and his father Nick Nolte in Ang Lee’s 2003 “Hulk.” Ultron takes the form of a robot and immediately downloads all of the information in the world from the Internet, quickly determining that world peace can only be achieved through — yikes! — the extinction of the human race.

The prospect of a sentient entity roaming the Internet at light-speed to gather information might seem more crazy and subversive if Scarlett Johansson (who plays Avenger Natasha Romanoff) hadn’t already done it twice since “The Avengers” — in Spike Jonze’s sublime love story “Her” and Luc Besson’s silly actioner “Lucy.”

On the other hand, James Spader‘s very recognizable voice gives Ultron more than sheer menace. He suffers from a kind of existential ennui, something each of the individual Avengers is dealing with as well. Constantly frustrated by the way humans think, he also has a really short temper. Even if Ultron’s logic is hard to follow, Spader gives the baddie an estimable soul, as he jumps from robot to robot.

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On a lighter note, “The Avengers: Age of Ultron” contains more blatantly cinematic fight scenes than the first film, and all kinds of creative examples of superhero teamwork. In “The Avengers,” Whedon explored what it would be like for these tremendous egos to meet for the first time. Now that they are working together, he pairs up weapons like Thor’s hammer and Captain America’s shield to create one dangerous, speed-of-light projectile. A moment like this does more than merely advance the action. It reinforces the characters of these boy scouts (Chris Hemsworth and Chris Evans respectively) from different universes, and Whedon expands this during the film, incorporating little moments of character into most of the action scenes.

Speaking of character: For all of the hype you’ll doubtless hear about how action-packed “The Avengers: Age of Ultron” is, let it be known that almost nobody in the superhero world can juggle an ensemble this big and still give each character a defining moment like Whedon. Bryan Singer figured this out in “X2,” but his latest X-Men movie “Days of Future Past” has at least five or six mutants whose names I can’t even remember.

Clint Barton (Jeremy Renner) has gotten short shrift in every Marvel movie until now, which perhaps explains why in “The Avengers: Age of Ultron,” he becomes the first Avenger to have a family. His homestead is pure old-school Americana, out in the country and cut off from all things modern and scary. Besides providing a sharply contrasting backdrop for these larger-than-life costumed heroes, it serves as a symbolic nod to the western, which is the genre the film fits into the most.

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Whedon is drawing from and reinforcing other classic themes throughout “The Avengers: Age of Ultron.” New superpowered characters like Scarlet Witch (Elizabeth Olsen) and Quicksilver (Aaron Taylor-Johnson) are out for pure revenge. A subplot between Hulk and Black Widow has echoes of “King Kong.” Because Whedon understands how these conflicts work, it’s kind of a disappointment that he isn’t given the complete freedom from a plot standpoint to really dig into them. Another new creation, the Vision (Paul Bettany), is barely given any explanation at all.

There are responsibilities, after all, in long-term episodic storytelling. The original cut of “The Avengers: Age of Ultron” was around three hours long, but it’s been pared down to two hours and 10 minutes, which seems more like a commercial decision than anything else. (It allows for more showtimes in a single day, and therefore bigger grosses.) This is completely understandable–and maybe three hours is too long to maintain a consistent pace–but with the new characters, all of their story arcs, and multiple overarching themes, I’d be very curious to see a film that gives those elements time to breathe.

Still, for an overstuffed spectacle, “The Avengers: Age of Ultron” looks inward more often than most.

“The Avengers: Age of Ultron” is showing everywhere in the universe and is rated PG-13 for intense sequences of sci-fi action, violence and destruction, suggestive comments, and semi-obscure pop culture references.