The exhibits at the Museum of Natural History put great weight in their educational value. To them, helping children learn could not be a higher calling.

A noble sentiment, of course, even when shared by a capuchin monkey and Attila the Hun. Yet it's too bad that Night at the Museum: Secret of the Tomb is itself so lightweight, only occasionally remembering the mission that drives its characters. Education doesn't have to hit you on the head with thudding obviousness, but it should be a little more than a thematic afterthought.

But as the movie opens, it's clear that spectacle is on its mind. The wealthy benefactors of the museum believe that its magic is the result of special effects -- never questioning that it only works at night. Nor does the script question how the spectacle it sets up would actually work, either. It just winds up set pieces and watches them go.

On this night, however, it goes awry as the magical tablet that brings the exhibits to life suddenly tarnishes. Characters turn mean, or rather, actually start behaving the way they historically did.

Museum guard turned "special effects wizard" Larry (Ben Stiller) has to then race against that tarnish to restore the magic and save his friends. It's a quest, if you will, that will take him to England and cross paths with Sir Lancelot (Dan Stevens).

In case you miss the meta-message, though, Larry's real quest is to take the next steps in fatherhood and let go of his plans for his son (Skyler Gisondo). If this is educational, it's a dangerous mixed message this movie sends to its most likely audience: college is for suckers. Go ahead and DJ on a mediterranean island. (Okay, maybe it's right, but still...)

The themes get dropped for long periods of action, because again, spectacle. Moving the action to the British Museum does mix things up a bit, and makes this perhaps the most fun in terms of kid-level creepy. Seeing broken Greek statues come to life for the first time is an indelible image, though for the most part, director Shawn Levy doesn't really play any of the possibilities out to their fruition.

Each sequence plays out gamely, but doesn't quite connect with anything else. Characters careen from threat to threat in order to make it seem like there's more plot than there is. (The actual secret of the tomb is pretty simple, and simply solved.)

So there has to be a fight with a dragon, because we have Lancelot. An ancient asian snake god will substitute nicely, and if you really want this to be an educational opportunity, you'll go home and teach your kids about Ray Harryhausen.

When this movie succeeds, it's strictly on the charisma of its actors. Stevens would make a good Lancelot in an actual adaptation of the King Arthur legend, but here the script requires him to be narcissistic at best, instead of really heroic.

Your mileage with Stiller may vary. He doesn't do anything here that breaks new ground in characterization, largely just repeating his usual tics and getting mocked by the others for it. Owen Wilson and Steve Coogan continue being fun as miniature warriors, though there's a strange vague running joke about Coogan's sexuality that is neither funny nor paid off.

And then, of course, there's the slight pall of this being Robin Williams' last live-action performance. An added poignancy floats over his performance. The movie might not be great, but Williams gave it a restrained all. When not being mawkish, the man played friendly warmth like few others, and his Teddy Roosevelt anchors the movie's messages. (Tributes to Williams in the future could do worse than use the clip of his last words here.)

As holiday entertainment, it's inoffensive. With a few extra cameos, it even has some fun moments. However, it just isn't much more than that, and maybe shouldn't have been expected to be as the third installment in a series that was just fine as a single movie. If we really want to teach kids something from the movies, how about knowing when it's time to leave the stage?

Derek McCaw