What a curious product this is. “The Secret Life of Walter Mitty” is star/director Ben Stiller’s big swing for the fences, his bid to deliver a whopper that defines him as a filmmaker of the first rank. It’s a swing and a miss.

The title character is very small, a milquetoast photo librarian for Life magazine. The movie, like Walter’s daydreams, is very big. It’s clearly angling for best picture, production design, cinematography and visual effects Oscars. There are action sequences that would fit comfortably into a Marvel superhero movie, and magnificent nature panoramas worthy of a Patagonia catalog. Every shot strains to be the most, the biggest, the ultimate.

The problem is, giving a comedy a blockbuster scale does not make it exponentially funnier. Stiller’s “Mitty,” ever-so-loosely based on James Thurber’s subversive short story, is a strenuous exercise in crowd-pleasing with the emotional heft of a “Hang in there” kitten poster.

Colorless Walter spends his days cataloging images from adventurous photographers. He’s too introverted to approach his office crush (Kristen Wiig, all sweet, easygoing charm). He’s too timid to confront the condescending new boss (droll, nasty Adam Scott) who is overseeing the magazine’s final print edition. He doesn’t even have the Dilbert-like gumption to fume resentfully. Walter is a nonentity to everyone but a legendary photojournalist (Sean Penn), who entrusts the humble clerk with the best picture of his career, the haunting image he declares will make the perfect farewell cover. The image that Walter, for the life of him, cannot find.

You will not be shocked to learn that chasing down the globe-hopping, devil-may-care cameraman to locate the lost photo transforms Walter. On intercontinental adventures that find him outrunning volcanic explosions in Iceland, wrestling sharks in the freezing North Atlantic and trekking the Himalayas, Walter evolves from a creature of fey pathos into a self-reliant mensch.

So far, so formula. The real surprise is that Stiller, who directed the sharp comedies “The Cable Guy,” “Zoolander” and “Tropic Thunder,” would release a movie this cloying and tonally wobbly. Walter’s daydreams have him leaping into exploding apartment buildings and battling his abusive boss on New York City streets like Spider-Man vs. the Green Goblin. Then the film downshifts to low-key domestic comedy with Kathryn Hahn and Shirley MacLaine as Walter’s ditsy sister and doting mom. There’s an odd throwaway sequence with Stiller and Wiig lampooning “The Curious Case of Benjamin Button,” then a musical interlude with Wiig serenading Stiller on guitar (a passage that at least advances the plot). Hope springs when Patton Oswalt makes an 11th-hour appearance, then dies as his character is denied even a dollop of wit.

The film never finds its footing, and viewers will be forgiven for losing track of which moments are fact or fantasy. What they will surely not lose track of is the aggressive in-story promotion of a certain pizza chain, online dating service, nationwide bank, pastry franchise and child’s toy, all forcefully inserted into the film regardless of context. It may be the film’s only memorable aspect. Mitty’s journeys to Timbuktu are altogether too cutesy-poo.