THE SECRET LIFE OF WALTER MITTY (Rated PG)

To say that Ben Stiller’s “The Secret Life of Walter Mitty” is based on the famous short story by James Thurber has a kernel of truth only insofar as the film’s titular character is, indeed, a socially awkward man who daydreams of performing heroic acts.

Pulling double duty as director and star, Stiller has been on both sides of the camera with films like “Tropic Thunder” and “Zoolander,” both of which were flat-out comedies. His work here is a more nuanced bittersweet comedy, slightly eccentric and reminiscent somewhat of “Greenberg.”

The daring exploits of Walter Mitty (Ben Stiller) bear little resemblance to the odd detours of Thurber’s version. The on-screen Walter’s lapses into fantasy are mostly geared to romancing a pretty co-worker or triumphing over danger with the passion and confidence of an action hero.

Whatever the reason for flights of fancy, Walter is a nebbish consigned to the increasingly unimportant task of managing the photo department of LIFE magazine, recently taken over by snotty corporate downsizing experts who are determined to shut down the print operation for good.

Sporting beards and sharply-tailored suits, the arrogant corporate types, led by the unctuous, detestable Ted Hendricks (Adam Scott), are hell-bent on eliminating many of the publishing empire’s jobs during its swift transition to the digital age.

Scrambling to revamp the media operations, management plans one final print issue of LIFE where the magazine’s cover image will carry great cultural significance. The assignment falls on Walter’s shoulders, a parting shot for the photo editor’s talent.

Out in the field and only intermittently reachable by phone or letter is noted war photojournalist Sean O’Connell (Sean Penn), who has shipped to Walter a set of negatives, one of which is deemed to be his best work ever.

The problem is that Walter is unable to find this mysterious negative in the roll of film that has arrived from an undisclosed location. Now he has to dodge the odious Ted Hendricks, trying to buy time in a frantic search for one lousy frame.

Meanwhile, Walter has developed a fruitless crush on lovely, clueless colleague Cheryl (Kristen Wiig), a single mother with a young son, who hardly takes notice of his clumsy, pointless efforts to initiate a conversation.

Walter also has to contend with moving his aging, meddling mother (Shirley MacLaine) into an assisted living facility, while enduring the taunts of his oddball sister (Kathryn Hahn).

Thurber’s Walter Mitty was a henpecked husband whose fantasies carried him away from the constraints of his marriage. Stiller’s version is a modern bachelor dreaming of romance and trying to make a connection on an Internet dating service.

There’s amusement in Walter’s attempts to create a profile for eHarmony, since he’s never done anything or been anywhere of interest. Walter fields the occasional phone call from his excited eHarmony advisor (Patton Oswalt) who tries to spur him into action.

What finally dislodges Walter from his reveries is not just looming unemployment, but the absolute devotion to his craft of running the photo department in all of its analog world glory. To that end, he must find the missing negative, in a hurry.

At this point, “The Secret Life of Walter Mitty” undergoes an almost complete tonal shift, as Walter must suddenly act in an unaccustomed role of being a doer rather than an inattentive observer on the sidelines.

Life for Walter must quickly take the shape of LIFE magazine’s famous motto, emblazoned on its building, which in part encourages people to “see things thousands of miles away.” It may be uncharacteristic for him, but Walter plunges into a reckless search for the elusive Sean O’Connell.

Following up on a few leads, Walter flies off to Greenland, only to find himself a few steps behind the famous photojournalist. From Greenland he ends up on a fishing trawler but not before having to swim with sharks in the ocean.

After the boat docks in Iceland, Walter’s next adventure takes him skateboarding in the direction of an erupting volcano, only to be rescued at the last moment by a passing motorist in a hurry to evacuate from the path of billowing smoke and lava.

The search must continue onto even more treacherous terrain as Walter makes his way to the remotest mountain region of the Himalayas, where finally he locates Sean O’Connell on the verge of capturing an image of a nearly extinct wild animal.

By this point, Walter is no longer a “Walter Mitty”-type character, having blossomed into an intrepid adventurer willing to go to great lengths in pursuit of a goal, becoming more like his idol Sean O’Connell.

“The Secret Life of Walter Mitty,” though clever and amusing, is not easily pegged as the standard comedic vehicle for Ben Stiller. Instead, it’s a humorous fantasy adventure layered with conflicting emotions, resulting in an enjoyable confection.

Tim Riley writes film and television reviews for Lake County News.