From its opening credit sequence, which finds Chris Pratt boogeying through an alien landscape to the strains of Redbone’s “Come and Get Your Love,” Guardians of the Galaxy seems the perfect antidote to Serious Summer Blockbuster Syndrome, a blissfully goofy and tartly self-aware slab of sci-fi silliness. Writer/director James Gunn is another of Marvel’s unconventional hires — his pitch-black 2010 comedy Super features Rainn Wilson as a mentally unbalanced would-be superhero who attacks people with a hammer while shouting “Shut up, crime!” — and for much of Guardians’ 122 minutes, Gunn’s grab-bag of quirky characters, genre ribbing, and incongruent ‘70s pop tunes goes over like gangbusters. The trouble is, it’s only irreverent to a point, and when the time comes to wrap things up, Gunn plays it strictly by the book. And this is becoming a real problem in the Marvel Cinematic Universe.

Two things should be clarified here. First, Guardians of the Galaxy is genuinely enjoyable. It’s got a sense of humor and a sense of fun too often lacking in big tentpole movies (and no, the “comic relief” of, say, a Transformers movie doesn’t count — more often than not, such dick-joke breaks are more depressing than amusing). Much of the picture is a showcase of the shambling comic ingenuity of star Pratt, who plays Peter “Star-Lord” Quill as a looser, hornier Han Solo, though he can barely keep Rocket Raccoon (voiced by Bradley Cooper) from stealing the picture out from under him.

The tone is comic, as well it should be — we’re talking about a movie where two of the main characters are a genetically modified, wisecracking raccoon and a monosyllabic tree creature. Gunn has fun sending up the conventions of peripheral characters (when tough guy Drax tells a would-be attacker, “I like your knife, I’m keeping it,” Gunn stays with the poor sap long enough for him to mutter, sadly, “That was my favorite knife…”), and he’s a got a knack for sly visual comedy (there’s a background gag involving a battery theft that produces a long, sustained laugh).

Most of all, he’s allowed to imbue the project with a kind of bouncy eccentricity — but only within the confines of the now-established Marvel formula. So that means we’ve got to sit through a bunch of frightfully dull exposition about the Kree and Thanos and Xandar and so forth, none of which actually matters (beyond making tenuous connections to other Marvel movies), because in the kind of roughhouse comedy Gunn seems to aspire to, the plot isn’t actually important beyond its capacity to serve as a clothesline for hanging comic sequences. (These scenes also make the puzzling error of hiring Lee Pace and then hiding him under layers of make-up, much as Thor: The Dark World wasted an unrecognizable Christopher Eccleston.) And that means we have to have a big action set piece at the conclusion of the second act, in which our heroes are defeated and all seems lost. And, most importantly, we have to have a giant, effects-heavy climactic battle, in which many things are blown up real good.

This brings us to my second point of order: I like these Marvel movies. The snoozy Thor flicks (Tom Hiddleston notwithstanding) aside, these are pop filmmaking of the highest order, from Captain America’s gee-whiz throwback charm to Iron Man 3’s spiky wit to Captain America: The Winter Soldier’s sly political satire to the honest-to-goodness character comedy of The Avengers. These movies are refreshingly funny, and they wear their idiosyncratic writers and directors’ authorial stamp with pride… but, again, only up to a point. Because no matter who is behind the wheel of a Marvel movie, and no matter what kind of great, left-field stuff they’ve done for the first 90 minutes, each film must conclude with the required CGI boom-boom fest. And with each passing film — but particularly with Guardians, which seems, until that beat arrives, hell-bent on confounding expectation—the obligatory third-act action climax seems more hollow and uninteresting.

Look, I recognize that they have a fanboy audience of young men who want nothing more than to watch things blow up (even if those things are just 1s and 0s, and look like it.) But these action climaxes aren’t what people like about these movies, or what make them memorable. When we remember The Avengers, we don’t think fondly of the numbing New York City smash-up or such generic action movie dialogue as “I’m bringing the party to you”; we think of the interactions between the characters during the film’s middle hour, and the screwball snap of Whedon’s dialogue. (The only memorable moment in that film’s climax is the unexpectedly uproarious beat between Hulk and Loki, and that’s because we’ve been primed for it; we spent an hour watching a comedy.) When we remember Iron Man 3, it’s for Shane Black’s delightful voice-over narration and the relationship between Tony Stark and little Harley Keener, not that endless fight between literally empty Iron Man suits. Captain America’s greatness was in its period style and the eye-popping transformation of Steve Rogers, not that dumb battle with Hugo Weaving (he peeled off his head or something, right? I can barely remember, and I saw that movie twice.)

And what’s great about Guardians is not the canon mumbo-jumbo or the conventional action/sci-fi beats, but when Gunn jettisons that nonsense and does his own, weird thing. When he gets to the required climactic battle, not only does our interest wander, but it feels as though his does too; his heart’s not in this stuff any more than Black or Whedon’s was, because they’re basically handing their movie, in the clutch, over to special effects wizards and second unit directors. But Guardians’ overall mock-comic tone makes the surrender to conventional blockbuster beats feel all the more arbitrary; as Joe Adamson once wrote about the climaxes of the Marx Brothers’ movies, “a film going nowhere in particular feels pretty sheepish about itself by the time it doesn’t get there.”

A couple of months back, in recognition of the dual anniversary of Gremlins and Ghostbusters, I wrote about how we don’t really see effects-heavy comedies of their ilk anymore. In Guardians’ first act, it felt like an honest contribution to that tradition, but put its climax up against, say, Ghostbusters’, and you’ll see the problem: in Ghostbusters, the effects are in service to the jokes, but in Guardians, it’s the other way around. From what makes it to the screen, the Marvel marching orders seem to be that you can tweak and kid all you like, but once you hit the 90-minute mark, there’s bills to pay. And eventually, that formulation’s gonna have to give if they want to keep their movies from becoming as wheezy and creaky as the rest of the Hollywood blockbuster “product.”