Walter Mitty is a timid man who escapes his mundane, unremarkable life by indulging in fantastic daydreams where he embodies a life of adventure and heroism. He works at Life magazine, handling precious photographic negatives and pining for his co-worker, Cheryl, whom he's too shy to approach. When it’s announced that Life magazine will cease publication of its print edition for a purely online presence, the final cover photo, captured by reclusive and adventurous photographer Sean O'Connell, is to symbolize the quintessence of Life magazine. However, Walter cannot find this photo, labeled as "Negative 25," and is spurred into action to recover it.

Walter's quest to find the elusive photographer and the missing negative takes him on an unexpected journey more extraordinary than any of his daydreams. From jumping into helicopter rides in Greenland with drunken pilots, to escaping erupting volcanoes in Iceland, Walter begins to live the kind of adventures he had only imagined. Meanwhile, his journey becomes as much about self-discovery as it is about the pursuit of the missing negative, shedding layers of the person he used to be and embracing the dynamic, brave individual he's becoming through real experiences that far surpass his fantasies.