“At just under two hours, Jack the Giant Slayer has more padding than a Mattress Warehouse.” —Robert Gifford

Jack the Giant Slayer, a multimillion-dollar, 3D fantasy-action-epic adaptation of the bedtime story “Jack and the Beanstalk,” will appeal to precisely two groups of people: children, and anyone who really, really wants to sit through a convoluted, 10-minute all-CGI explanation of how magic beans came to be magic.

The answer: Well, there were these monks, and they really wanted to meet God, so they grew a giant beanstalk they hoped would bring them to heaven, Tower of Babel-style. It ended up leading them to a floating land filled with evil giants … and, oh, it’s really too stupid to be worth explaining.

Director Bryan Singer (Valkyrie), the man behind the first two X-Men movies, and the four screenwriters (evidently it is very hard to come up with lines such as, “Piss off, giant!”) have the unenviable task of extending a 15-minute story designed to lull children into sleep into a feature-length blockbuster. They are, unfortunately, defeated by that task; at just under two hours, Jack the Giant Slayer has more padding than a Mattress Warehouse.

So, the titular Jack (Nicholas Hoult, Warm Bodies) becomes a Joseph Campbell-for-dummies cardboard cutout of a hero. And instead of merely getting his ass eaten by giants (or nearly so, anyway), he gets caught up in a centuries-old, apocalyptic struggle between men and giants involving magic MacGuffin crowns, palace intrigues and a romance with a princess (Eleanor Tomlinson, Alice in Wonderland). It’s all very silly.

“Silly” was perhaps an inevitability — it’s a bloated Hollywood adaptation of a fairy tale, after all. Case in point: There are giants named Fee, Fye, Foe and Fumm. How clever!

What’s truly damning is how boring all of this is. For a movie that promises giant-slaying in its title, it sure takes Jack the Giant Slayer a long time to actually get to any, you know, giant-slaying. An ungodly amount of time is spent on exposition, power struggles within the palace, climbing up and down various beanstalks and then, worst sin of all, power struggles within the giant community. No one — barring the very young, the very forgiving or the very stupid — is interested in the politics of giants.

I am curious to know why the giants all seem to be dudes, however. Where do baby giants come from?

It’s only in the final 20 minutes that the giants even become a threat to the human kingdom, which saps everything prior to that of any energy — though, oddly, the giants threaten to conquer the “Viking myth of a land beyond the sea,” because apparently Hollywood has grown so jingoistic that even a film set in a generic medieval fantasyland needs a threat to the American homeland.

Once the film actually gets down to giant-slaying, it picks up a bit — a climactic siege, centered around a chaotic game of tug-of-war, is competently staged and modestly exciting — but it takes so long to get there and resolves so quickly and neatly that it feels bloodless and devoid of urgency.

The film’s other sins are too numerous to list, but among the gravest are: casting Hoult and Tomlinson, a pair with the charisma and chemistry of computerized customer service representatives, as the leads and then weighing them down further with dialogue so stilted, Daniel Day-Lewis couldn’t redeem it; allowing talented actors such as Ian McShane (Snow White and the Huntsman), Ewan McGregor (The Impossible) and Stanley Tucci (Gambit) to do nothing but blandly deliver exposition; making the princess into a feminist who thinks her gender shouldn’t limit her, then forcing her to spend most of the movie waiting to be rescued by a man.

Oh, and also, at one point a giant eats his booger. That about covers it.