"Night at the Museum: Secret of the Tomb" is reportedly the third and final installment in a family fantasy series that started with 2006's "Night at the Museum" and continued with 2009's "Night at the Museum: Battle of the Smithsonian." The first two films were no great shakes, but passable time-wasters with a hint of charm to go along with a lot of schtick. This third pic dares not diverge from formula, but it is a more tiring, pedestrian affair. Having attended a screening just days prior to this writing, its memory has already begun to fade. With the exception of a winsomely sentimental denouement, this is exceedingly humdrum stuff.

In 1938 Egypt, an expedition into an ancient underground tomb led to the discovery of the powerful Tablet of Ahkmenrah. Sticking their nose up at warnings that the end will come if this artifact is disturbed, they promptly take it. Over seventy-five years later, it now resides in New York City's Museum of Natural History, a place that night watchman Larry Daley (Ben Stiller) is well aware has magical powers of its own. When the tablet begins to mysteriously erode and starts adversely affecting his friends—that is, the historical-figure exhibits come to life—he and teenage son Nick (Skyler Gisondo) travel to The British Museum in London in hopes of talking to Merenkahre (Ben Kingsley), a pharaoh who knows just how to reverse the curse.

"Night at the Museum: Secret of the Tomb" smells like the second sequel to a franchise that never intended on becoming a trilogy. Indeed, the whole thing has a tacked-on feel to it, one that director Shawn Levy and screenwriters David Guion and Michael Handelman (2010's "Dinner for Schmucks") are not able to assuage until the film has very nearly run its course. The plot is a simplistic throwaway, as are uninspired set-pieces involving falling through air ducts and a lava-flow attack on miniature cowboy Jedediah (Owen Wilson) and pint-sized Roman soldier Octavius (Steve Coogan) that is ultimately put out by a monkey's urination spray. There is also a chase/skirmish inside an M.C. Escher drawing—an idea that would have been innovative if it hadn't already been done better in 1986's "Labyrinth" and 1989's "A Nightmare on Elm Street 5: The Dream Child." Save for the bits featuring The British Museum's cheerfully talkative security guard Tilly (Rebel Wilson), cute cameos from Hugh Jackman and Alice Eve, and some unexpectedly random film references to "Psycho," "Dirty Dancing" and, believe it or not, the 2009 dolphin documentary "The Cove," comic inspiration is decidedly flaccid.

Coming off of his no doubt much more personally and creatively satisfying 2013 directorial effort, "The Secret Life of Walter Mitty" (a film that gets better with each viewing), Ben Stiller is back in paycheck mode with "Night at the Museum: Secret of the Tomb." This is not at all a slight to the actor, who is winning as Larry and manages to bring a modicum of respect to the project, but it is difficult to watch this knowing that he could be expending his efforts on so many more worthwhile pursuits. Ever the scene-stealer, Rebel Wilson (2012's "Pitch Perfect") gives her scenes the burst of energy for which the rest of the movie yearns. Granted, hers is a relatively thankless part, but if there absolutely had to be a fourth "Night at the Museum," she should be the go-to choice to carry Stiller's torch.

Tiresome and inconsequential to a fault, "Night at the Museum: Secret of the Tomb" finally starts to work. The only problem? There are less than ten minutes left of the running time. For a role that has wasted his talents for three entries, the late Robin Williams (2014's "The Angriest Man in Brooklyn"), as museum exhibition figure Theodore Roosevelt, at long last gets a touching closing moment in the spotlight. Williams' concluding scene, playing on two different poignant levels, sends the beloved artist off on a note that perfectly encapsulates his effortless, career-long ability to move between humor and pathos. If there is one thing "Night at the Museum: Secret of the Tomb" is good for, it is this. Wading through the bulk of the other 90 minutes to get there is a decidedly more cumbersome chore.