Five years ago, Iron Man blasted into theaters, bringing a whole new sensibility and vitality to comic-book movies. Iron Man opened the door not only to a new franchise, but a whole comic-book universe of Marvel heroes, culminating in last summer’s triumphant mega-blockbuster The Avengers.

That’s a lot for Iron Man 3 — the first post-Avengers film in Marvel movie continuity — to live up to. (The Amazing Spider-Man, which opened last July, doesn’t count, since it’s a Sony film outside the Disney-owned Marvel cinematic universe.)

At this point, there’s no doubt that Robert Downey Jr. is up to the challenge. His patented blend of self-deprecating arrogance and nervous insouciance has made Tony Stark the most vividly rendered big-screen superhero of all time.

Downey is up to the challenge — but is Tony? The Avengers’ business catapulted him into a whole new league: Asgardian demigods, outer-space alien armies, an enormous green rage monster and much more. In the end, he almost died on the threshold of another universe. What kind of toll does that take on a man, even a "genius billionaire playboy philanthropist" in a shiny metal suit?

Even Gwyneth Paltrow’s Pepper Potts is feeling the strain of trying to ground Tony. They’re living together now, but at times Tony seems more, um, distant than ever before. He isn’t sleeping, and he’s obsessively working in his basement laboratory building armor.

If all this seems potentially grim and Dark Knight-ish, the snappy banter and jokey humor of the past installments is still in evidence ("Einstein slept three hours a year, and look what he accomplished"). That’s courtesy of co-writer and director Shane Black (a veteran screenwriter with only one other directorial credit, Kiss Kiss Bang Bang).

All this is a potentially promising setup for a slam-bang finale to what has been, despite its flaws, one of the brightest and most entertaining franchises around. Unfortunately, the plot is pretty much a disaster. A string of miscalculations hamper the fun.

And a late revelation, when you stop and think about it, undermines most of the preceding drama.

Like Iron Man 2, the "threequel" gives us two villains who turn out to be connected. One is a terrifying terrorist mastermind named the Mandarin (Ben Kingsley), who is attacking targets on American soil and threatening to kill the president of the United States. The other is a scientist named Aldrich Killian (Guy Pearce), who is working on a bioresearch project to create a serum that’s supposed to "hack" the human genetic code and, like, improve it or something. (Both of these characters come from the comic books, as does the "Extremis" serum and its startling effects.)

Alas, the Mandarin is completely undermined in an anticlimactic revelation, a chicken-hearted cop-out presumably intended to help the movie play well in China, one of Hollywood’s biggest foreign markets.

The original Iron Man was willing to deal pretty honestly, within its comic-book milieu, with the threat of foreign terrorism. The terrorists in its Afghanistan caves were an international lot with no discussed religious or political agenda, but their methods and behavior were plausible enough, and there was a certain thoughtfulness to the political subtext of Iron Man touching down in the Middle East to clean up a problem that he had inadvertently created with his own weapons.

Compared to that, what happens in Iron Man 3 is a travesty — one in which the U.S. government is implicated at (almost) the highest level. As it happens, this cop-out plays particularly poorly after the Boston Marathon bombings and the inevitable, inflammatory "truther" allegations of a government-orchestrated setup or inside job.

Then there’s Killian, who’s ultimately the real villain, and his Extremis serum.

For the uninitiated who haven’t read the comic-book series on which the movie is based (which, for what it’s worth, includes me), the effects of the serum seem kind of … random.

Normal-looking humans possess startling regenerative powers that would put the X-Men’s Wolverine to shame. This is a problem, because there’s essentially no way to incapacitate these people, so you basically have to kill them to stop them. All the killing puts a damper on the escapist fun quotient — and that’s apart from Extremis-powered humans going haywire and literally exploding with such force that bystanders are vaporized.

The super-baddies also have the ability to generate enormous amounts of heat, melting steel with their bare hands (a little Human Torch thrown in). One of them even breathes fire. Oh, they do electricity, too. One of them shorts out Tony’s armored ally Jim Rhodes (Don Cheadle), War Machine (or "Iron Patriot," as the government has rebranded him). This makes no sense. In The Avengers, Thor hit Iron Man with a lightning bolt, and it charged up the suit to "400% capacity." Now an electrical zap shuts down the armor. How does that work?

Then there’s Tony’s newest armor. It seems each movie must reveal more high-tech coolness, but at this point they’ve improved the armor to the point where it’s basically magic. The armor now has the power to assemble itself around him, each individual piece flying under its own power, ostensibly obeying tiny computer chips embedded in Tony’s arm, but seemingly obeying his very thoughts. He can command them to do other things too — assemble around someone else, for instance.

In the past, we accepted the crazy things the suit could do because it was all powered by the "arc reactor" in Tony’s chest. Now we’ve got individual bits of armor — gauntlets, boots, shoulder thingies — flying around at Tony’s command, sometimes miles at a time.

In one scene, Tony does battle wearing only one glove and one boot, jetting a bit and firing repulsor rays. The units aren’t connected to anything, so where’s the power coming from?

The magic armor might not be as big of a problem if it weren’t for the fact that Tony goes for almost half the movie with the magic armor in a busted-down state (shades of The Dark Knight Rises after all), battling super-powered foes and breaking into fortified terrorist compounds as Tony Stark … and then, at the climax, he suddenly pulls an ace from his sleeve — no, not an ace, a whole royal flush, and then some — that makes a joke of the last hour of the movie. It’s far from the only plot hole, but it’s easily the most glaring.

Despite all that, Iron Man 3 is still entertaining. Downey’s charisma and energy continue to power the franchise; when he says, as he did in the last shot of the first film, "I am Iron Man," it’s Downey talking as much as Tony. But there’s a problem at the movie’s heart, which, as always, is Pepper. Pepper has always been what grounded Tony, what connected him to reality. Here, suffice to say, the movie disconnects Pepper from reality, which is just not right. Whether or not Tony deserved better, Pepper certainly did.

Steven D. Greydanus is the Register’s film critic and editor and chief critic at Decent Films.

He is currently studying for the permanent diaconate for the Archdiocese of Newark, New Jersey.

Content Advisory: Much comic-book action and mayhem, including images of human beings exploding and killing others; some innuendo and mild rude humor; a depiction of a live-in relationship; some objectionable language. Teens and up.