Ever since the box-office success of Spider-Man in 2002, summer movies have been dominated by superheroes like the Hulk, Iron Man and Batman.

A.O. SCOTT ESSAY

“Batman has no limits,” says Bruce Wayne to his manservant, Alfred, early in “The Dark Knight,” and the accountants at Warner Brothers, which released the movie, are likely to agree. I’m not so sure.
“The Dark Knight,” praised by critics for its somber themes and grand ambitions, has proven to be a mighty box office force in a summer already dominated by superheroes of various kinds.
But any comic book fan knows that a hero at the height of his powers is a few panels removed from mortal danger, and that hubris has a way of summoning new enemies out of the shadows. Are the Caped Crusader and his colleagues basking in an endless summer of triumph, or are they already on their way out?
The season began with “Iron Man” back in May, which anticipated “The Dark Knight” in striking many reviewers as a pleasant surprise . The first weekend in July belonged to “Hancock,” which played with the superhero archetype by making him a grouchy, slovenly drunk rather than a brilliant scientist, a dashing billionaire or some combination of the two. In that case, the reviews were mixed, but the money flowed in anyway. Even the lackluster “Incredible Hulk,” back in June, managed a reasonably robust opening, as did “Hellboy II,” a somewhat more esoteric comic-book movie.
The commercial strength of the superhero genre is hardly news, of course. Ever since the success of “Spider-Man” back in 2002, this decade has been something of a golden age for large-scale action movies featuring guys in high-tech bodysuits battling garishly costumed, ruthless criminal masterminds. Some of them - the “Fantastic Four” pictures, most notably - are content to be entertaining pop-culture throwaways. But most aspire to be something more, to be taken as seriously as their heroes and villains take themselves.
There have been disappointments - Ang Lee’s 2003 “Hulk”; Bryan Singer’s “Superman Returns”; the third installment of the “X-Men” series, directed by Brett Ratner - but these have hardly dented the power of the genre.
Still, I have a hunch, and perhaps a hope, that “Iron Man,” “Hancock” and “Dark Knight” together represent a peak, by which I mean not only a previously unattained level of quality and interest, but also the beginning of a decline. In their very different ways, these films discover the limits built into the superhero genre as it currently exists.
I’m willing to grant that “The Dark Knight” is as good as a movie of its kind can be. There is no doubt that Batman provided Christopher Nolan with a platform for his artistic ambitions.
You can’t set out to make a psychological thriller and expect to command anything like the $185 million budget Mr. Nolan had at his disposal in “The Dark Knight.” And that money allowed Mr. Nolan and his team to create a seamless and evocative visual atmosphere .
But to paraphrase something the Joker says to Batman, “The Dark Knight” has rules, and they are the conventions that no movie of this kind can escape. The climax must be a fight with the villain, during which the symbiosis of good guy and bad guy, implicit throughout, must be articulated. The end must point forward to a sequel, and an aura of moral consequence must be sustained even as the killings, explosions and chases multiply.
Every movie genre is governed by conventions, and every decent genre movie explores the zones of freedom within those parameters. Thus “Iron Man” loosens the reins to give Robert Downey Jr. room to explore the kinks and idiosyncrasies of Tony Stark, the playboy billionaire engineering genius who grows up and builds himself a metal suit. And “Hancock” takes the conceit of a dissipated, semi-competent hero and turns it into the occasion for some sharp satire on race, celebrity and the supposedly universal likability of its star, Will Smith.
But in both cases, as soon as the main character is ready to do battle, the originality drains out of the picture, and the commercial imperatives - the big fight, the overscaled action extravaganza - take over.
The disappointment comes from the way the picture spells out lofty, serious themes and then … spells them out again. What kind of hero do we need- Where is the line between justice and vengeance?
And yet stating such themes is as far as the current wave of superhero movies seems able or willing to go. The masked and caped crusaders must turn big profits on a global scale and satisfy an audience hungry for the thrill of novelty and the comforts of the familiar.
Is it just me, or is the strain starting to show?