Based on the short story by James Thurber (well, sort of), The Secret Life of Walter Mitty is the story of a man whose dull, unextraordinary life leads to a series of complicated, heroic daydreams. In the original short story, that's as far as it goes. Walter's life does not change dramatically; he still feels the need to escape into his secret lives.
The film version, directed by and starring Ben Stiller, starts much the same way. Here, Mitty is a quiet, unassuming Life magazine employee, developing photographs and experiencing the beauty of the world vicariously. In his dreams, he always has the perfect comeback (or the perfectly placed punch) for his bullying workmate (Adam Scott), he always heroically wins the attention of the object of his affection (Kristen Wiig), his life is always bigger and grander and more beautiful. In the real world, of course, his life couldn't be duller. There's even a running storyline with an eHarmony employee (Patton Oswalt) doing his best to build Walter's profile into one interesting enough to garner even the slightest bit of attention.
When Life announces it is ceasing its print editions and laying off most of its employees, freelance photographer Sean O'Connell (Sean Penn) requests that his greatest work be featured on the cover of the last issue. The only problem is, the negative is missing. Deciding to chase Sean down to recover it, Walter draws inspiration from Life's motto, "To see the world, things dangerous to come, to see behind walls, draw closer, to find each other, and to feel. That is the purpose of life." And so his own adventure begins.
The Secret Life of Walter Mitty is at its best, both emotionally and visually, during Walter's early fantasy sequences. The film sets high expectations for itself that it never quite reaches once Walter steps away from fantasy and into real adventures. Veering too far into sentimentality and coincidence don't mesh with a story whose origin is so bittersweet. It's a nice movie that had the potential to be a great movie, but never reaches it.