There's not much point in watching "The Avengers" movie if you haven't seen or aren't at least familiar with the other Marvel movies in this series, which began with the first "Iron Man" in 2008. It's the previous movies that do the dirty work of introductions and establishing character arcs for the superheroes Iron Man (Robert Downey Jr.), Captain America (Chris Evans), and Thor (Chris Hemsworth). None of them really get much development in "The Avengers," as most of the brief moments for character building are reserved for the team members we haven't really gotten to know yet: supersoldier Nick Fury (Samuel L. Jackson), deadly spy Black Widow (Scarlett Johansson), dead shot archer Hawkeye (Jeremy Renner), and the new Bruce Banner/Incredible Hulk (Mark Ruffalo).
In two and a half hours, "The Avengers" never slows down, but then again it can't really afford to. Director Joss Whedon has the unenviable task of juggling seven primary characters, at least as many secondary players, and maneuvering everyone into position for the great big team-up and showdown that Marvel fans are lining up to see. The giant world crisis that starts the party is a fairly generic alien invasion. Thor's megalomaniacal brother Loki (Tom Hiddleston), last seen being soundly defeated by the god of thunder in last year's "Thor" feature, has returned with some new toys, intent on unleashing an army of newfound alien allies on the human race, and conquering the planet. The first step is stealing a supernatural energy source called the Tesseract, a big glowing MacGuffin cube, from Nick Fury's espionage agency, S.H.I.E.L.D., and brainwashing a few scientists and Hawkeye into becoming Loki's new minions.
In a movie that exists entirely to take advantage of all these different superheroes existing in the same universe, and that has almost no effect on any of their stories individually, the pleasures are two-fold. On the one hand, it's a treat to see all these different characters meet and interact, throwing one-liners at each other and having some contentious early skirmishes before finally finding their team dynamic. And then we get to watch them go to work, getting into fights with Loki and the alien invaders. The last act of the film is one giant battle sequence in downtown Manhattan, and an absolute joy to watch when we finally get there. But it's the getting there that's the tricky part. A lot of these characters are bringing a lot of baggage with them, and answering the lingering questions and accounting for all the loose ends from "Captain America" and "Thor" takes up a lot of time.
Whedon isn't entirely successful at getting all his ducks in a row. Captain America is clearly in the middle of a bumpy adjustment to the modern world and acting more hostile than usual, which is acknowledged, but has no effect on what's going on. Thor has the most personal stake in Loki's machinations, and the writers make sure that he gets his one-on-one scenes with his brother, but there's no time to get into any depth. Bruce Banner gets a very small character arc. Tony Stark gets one too, if you're paying close attention. Surprisingly, the one character who has the closest thing to a full subplot turns out to be the Black Widow, who is perhaps more concerned with getting Hawkeye out of Loki's clutches than saving the world. Even though the story is as simple as it possibly could be, there's still the constant feeling that there's too much going on and too much to keep track of.
But when the picture works, it works. The big, wild comic book spectacle is often senseless but always satisfying, and Whedon keeps finding ways to add those important little character moments, making sure that everyone gets their moment in the sun and their share of the good quips. Zak Penn gets a writing credit, but any Joss Whedon fan will recognize his dialogue and humor in a second. Nobody gets enough screen time in an ensemble this large, but all the performances are solid, and there is exactly the right proportion of Tony Stark behaving like an ass and the Incredible Hulk reveling in carnage and Loki grinning in villainous anticipation.
If you are a fan of the previous movies, it's worth seeing "Avengers" for the payoff for all the unexplained cameos and odd bits of extraneous plotting that were shoehorned into "Thor" and "Iron Man 2," in order to set up this movie. Sitting through the first half can be tedious, trying to keep up with the minutiae of the a plot that really doesn't matter much in the end. But when the action kicks into gear, and the Avengers are fully loosed on their enemies, you couldn't ask for a better way to start the blockbuster season.