These days, Iron Man is just a cog in a machine, one of several pieces being put into place by Marvel for the company’s upcoming movie version of The Avengers, in which several of its most recognizable superheroes will team up. While two years ago the high-tech armored superhero debuted as the star attraction in Marvel’s new slate of in-house movies, now he’s in danger of being overshadowed by other superheroes—even in his own movie.

So Iron Man 2 is a transitional piece, a movie that’s trying to be several things at once: Most successfully, it’s a complete story about cocky multimillionaire playboy Tony Stark (Robert Downey Jr.), also known as Iron Man, taking down Russian villain Whiplash (Mickey Rourke), as well as Stark’s nefarious corporate rival Justin Hammer (Sam Rockwell). It’s also an attempt to deepen the Iron Man mythology, with a long and ultimately fruitless subplot about Stark’s life-saving battery implant (which also powers the Iron Man suit) slowly killing him.

And it’s hard to say yet how successful Iron Man 2 is in its least prominent but in many ways most important task: setting things in motion for the eventual Avengers film. Samuel L. Jackson as Nick Fury, Scarlett Johansson as the Black Widow and Don Cheadle (replacing Terrence Howard) as War Machine are all featured substantially. With all these larger-than-life crime-fighters getting screen time, it’s a wonder the movie has room left for Stark.

Director Jon Favreau and screenwriter Justin Theroux balance things better than you might expect, though, and Iron Man 2 carries on the sense of goofy fun established by the original. Downey remains charismatic and clever, winking at the audience just enough so that you know he doesn’t take this stuff too seriously, but never going too far into camp or self-parody. The movie starts out well with the ominous origin of Rourke’s Whiplash, Stark grandstanding at a Senate hearing and an Iron Man/Whiplash battle in the midst of an auto race in Monaco.

Then things slow down for way too long, as the action subsides and the subplots grind into gear. Stark’s self-destructive antics are played too broadly, and the way his tech is supposedly turning against him never feels as serious or important as it should. The relationship between Stark and his assistant, Pepper Potts (Gwyneth Paltrow), one of the first movie’s strongest elements, falters a bit here.

But just when it seems like the movie has lost its way, things come together beautifully for the climax, an exciting action sequence that makes good use of those extra superheroes, although it also highlights just how superfluous Whiplash is to the overall story. There’s too much superfluity for Iron Man 2 to be a great comic-book movie, but when it’s good, it’s exactly the kind of summer entertainment people are looking for. As for its function as a precursor to a bunch of movies that have yet to be made? If 2012’s The Avengers turns out to be awesome, then all will be forgiven.