Walter, a negatives assets manager at Life Magazine, has a colorful imagination, one in which he is always taking risks and getting into outrageous adventures. At work and in life, however, he is a mild-mannered doormat, who goes along with the stronger personalities in his life and fulfills his obligations dutifully.
The magazine is downsizing and getting ready to produce their last-ever print issue. Renowned photographer Sean O'Connell has taken a series of photographs for the cover, and sends Walter the gift of a wallet to thank him for his work over the years, along with a negative of the photograph he wants to grace the cover. The only problem is: the negative is missing. Walter stalls his manager for time to locate it. Meanwhile Cheryl, Walter's office crush, has an idea that they can use the other negatives as a clue to Sean's current location and track him down. When Walter decides that Sean must be in Greenland, he books a plane ticket there to find Sean.
Walter arrives in Nuuk, the capital city of Greenland, and meets a local bartender who tells him that Sean has already left by ship. In order to find him, Walter tries to get on the postal helicopter that delivers mail along the coastline. The pilot of the helicopter is drunk, but he is wearing the distinctive ring that Walter spotted in one of Sean's picture. Out at sea, Walter jumps out of the helicopter and into the shark-infested waters, where he nearly gets eaten but is pulled onto a postal boat at the last moment.
Walter's timing is terrible yet again, as Sean has already left the boat. When the men on the boat give Walter some clementine cake that seems to be identical to the kind Walter's mother makes, Walter notices a travel itinerary on the wrapping of the cake. Sean is headed to Iceland to photograph a volcano there.
In Iceland, Mitty bikes to the volcano, and eventually gets a longboard from two Icelandic teenagers, which he uses to get closer to the volcano. He spots Sean on a little plane flying close to the volcano mouth. As he goes to follow, the volcano erupts, and Walter has no choice but to flee, losing Sean yet again. He returns to New York.
Back in New York, Walter is fired, and deduces that Cheryl and her estranged husband are back together. He leaves the Icelandic longboard on Cheryl's stoop as a gift for her son, Rich. Despondent, he goes to visit his mother, and throws the wallet that Sean gave him in the trash. Sitting on his mother's couch, he recognizes that her piano is actually the curved wood in one of Sean's negatives. His mother tells him that she has met Sean before, and that she told him all about it, but as usual Walter was too busy daydreaming about something else to listen to her.
Walter tracks Sean down in the Himalayas, photographing a rare snow leopard. He asks him about the missing negative, and Sean tells him that it was in the wallet he gave Walter as a gift. When Walter presses Sean to tell him what the photograph depicts, Sean refuses. Walter flies to Los Angeles but is detained at the airport. The only person he knows in Los Angeles is Todd, a customer representative from eHarmony, with whom Walter has been in touch about his dating profile.
Walter's mother decides to sell her piano, and Walter accompanies her. When he casually recounts the story of the mysterious negative in the wallet, his mother tells him that she rescued the wallet from the trash as soon as he had left. Sure enough, there is a tiny negative-sized envelope inside the wallet. Triumphant, Walter takes the negative to the magazine offices, and gives it to the manager who fired him, Ted. He then tells Ted off for disrespecting his tenured and loyal employees.
Later, Walter runs into Cheryl and is finally bold enough to tell her how he feels about her. She tells him her ex-husband was only at her house repairing the refrigerator, and they're not back together. Walter tells Cheryl about his adventure and about finding the negative, although he still doesn't know what was on it. Walking to the subway, they pass a newsstand which is selling the final print issue of Life. The issue is dedicated to its staff, and the cover shows Walter sitting outside the Time-Life building studying a contact sheet. As they walk down the street, Walter and Cheryl hold hands.