As the film opens, we see Walter Mitty sitting at a desk in his apartment reviewing his finances. He searches for "Cheryl Melhoff" on eHarmony, a dating website, and clicks on her profile. On it, she has filled out a section about her "perfect man," which reads, "Adventurous, Brave, Creative (or employed)." Her likes read, "My sister" and "mystery novels," and her dislikes read, "My sister" and "internet dating." Under "You should know," it says that she has a three-legged dog and a child. Walter laughs at her profile, then goes to send her a "wink," but decides against it, walking away from the desk. He then impulsively sends her a wink, but a notification appears that says, "Unable to perform request." Every time he tries to send a wink, the computer refuses to send it through. He gives up and leaves his apartment building.
At 125th Street in Manhattan, Walter waits for a subway to take him to work. Sitting on a bench, he calls a representative from eHarmony and tries to solve the wink problem. He tells the representative that he is trying to send a wink to Cheryl, a woman who is new in his office, and whom he heard telling someone that she was on eHarmony at the bagel station. The representative tells Walter that part of the issue has to do with the fact that Walter has not filled out the "Been There, Done That" section of his profile. "Have you done anything noteworthy, mentionable?" the representative asks.
Suddenly, Walter runs down the subway platform, jumps into a nearby building, and rescues a dog, yelling for people to get out of the apartment before it blows up. It turns out that the dog is Cheryl's three-legged dog. She runs out after Walter and asks him how he knew the building was going to catch on fire. He tells her he heard barking and could smell gas, before handing her a prosthesis for the dog.
"God you're noteworthy!" Cheryl says, to which Walter replies that he lives "by the ABC's: adventurous, brave, creative." Suddenly, Walter is jolted out of his fantasy of heroism and hears the customer service rep on the other end asking if he's still there. The rescue was all a giant fantasy, and his train has just left the platform without him. Abruptly, Walter hangs up the phone and runs to get to work.
Arriving at work at the Time & Life Building, a coworker tells Walter that their company just got "acquired." They are interrupted by the arrival of Walter's kooky sister, Odessa, who wishes him a happy 42nd birthday and hands him a clementine cake that their mother made him. "Aren't you supposed to be at the retirement place getting the piano moved in?" he asks her, but Odessa insists that the piano is almost in the retirement home, but that she got a call about an audition and needs him to go finish moving the piano in.
Walter tells Odessa he's late to work and cannot go, and Odessa tells him that she has to go to an audition to play Rizzo in a production of Grease. "Don't go to Rizzo!" Walter says, going into work and leaving her in the lobby. As two of Walter's coworkers complain about the fact that their company got acquired, a manager of the transition, Ted Hendricks, approaches and introduces himself. When he asks Walter what he does at the company, Walter has a fantasy of making fun of Ted's beard and making his coworkers laugh heartily.
Walter is jolted out of his fantasy by Ted asking him again what he does. "I'm a negative asset manager," Walter finally says, and Ted scoffs. Walter goes to his office, carrying the clementine cake. He spots Cheryl, who appears to be smiling at him and asking him how his weekend was. Walter lights up and begins to answer her, when it becomes clear that Cheryl is talking to another woman in the office.
As Walter pours himself a cup of coffee, he eavesdrops on Cheryl's conversation. Cheryl makes a joke about having an exciting weekend taking an arctic bath with her lover, before revealing that she actually just waited for her refrigerator repairman and then he came.
Walter suddenly has a fantasy of being the suave and seductive lover that Cheryl described in her joke. He imagines a whole arctic world and meeting. Back in reality, Ted and some other coworkers are standing near Walter and making fun of the fact that he hasn't moved in a while. Ted flicks a paper clip at Walter, which brings him out of his fantasy haze.
Down in his lower-level office, Walter greets Hernando, his coworker, who tells him that the acquisition representatives won't be able to evaluate their work given its intricate nature. On Walter's desk there is some mail from a photojournalist he works with, Sean O'Connell. O'Connell has sent him a canister with negatives, and a gift of a wallet in appreciation of his work over the years. A note with the gifts reads, "Heard rumblings Life's done. Wanted to say thanks. Take a look inside. A gift for all the years of hard work. Sorry about the neg roll. I spilled some blood on it while self stitching a gun wound to my abdomen, but number 25 is my best ever. The Quintessence of life, I think I trust you'll get it where it needs to go. You always do."
On the wallet, there's an inscription of the magazine motto that reads, "To see the world, things dangerous to come, to see behind walls, to draw closer, to find each other and to feel. That is the purpose of life." Walter smiles at the inscription, then looks at the negatives, but finds that one of them is missing. Suddenly, a coworker appears and says there's a floor meeting and that Walter and Hernando should bring negative 25, the negative that's missing, to it.
As the coworker leaves, Hernando and Walter look at each other, worried. Walter tells Hernando to make a contact sheet of the negatives and not to tell anyone about the missing one. When Walter arrives late to the floor meeting, his boss is announcing that the coming month's issue of the magazine is going to be its last. He then tells the employees that they will be downsizing in the coming weeks as well. Ted takes over and says that they will be publishing the very last issue of Life magazine, and that Sean O'Connell, who has never been willing to communicate with the executives at the magazine, has sent them a telegram instructing them that they should use negative 25 for the cover of the last issue.
Ted asks Walter for the negative, but Walter tells him that it's being processed and will be ready for the magazine publication in two-and-a-half weeks. On his way back to his office, Walter runs into Cheryl and introduces himself. When she alludes to the fact that he's handling the last cover photo, Walter asks her if she has Sean O'Connell's address, and she tells him he's hard to pin down, but offers to check with another coworker about it.
Downstairs, Walter examines the contact sheet and looks at negatives 26 and 27. Outside later, Cheryl walks up to Walter and tells her she's looked into finding Sean O'Connell's address, before admiring the glasses he uses to look at the negatives. When she sits down and asks about the negatives, Walter confides in her that they cannot find negative 25 and that he's looking for Sean.
Cheryl tells Walter that she takes a class on writing mystery novels, and that the key is working backward. They examine the negatives around 25, which depict a thumb, a body of water, and an unidentifiable curved object. As Cheryl goes back into the building, Walter has a fantasy about telling Cheryl that he's been working on something creative and then gesturing towards a large gold statue of the two of them embracing.
Walter suddenly gets a call from Odessa telling him that their mother's piano won't fit in her apartment. They tell the apartment manager that the piano was a gift from their father to their mother and that it's important. He shows them another apartment, which they agree to rent.
Odessa pulls out an action figure, "Stretch Armstong," and presents it to Walter as a birthday present. In their mother's apartment, Odessa finds a backpack that Walter was going to use on a backpacking trip the year their father died, and their mother tells them that it's in a box filled with things of Walter's.
Before we have learned much about the setting and scenario that contextualizes the film, we get an intimate window into the dating life of the protagonist, Walter Mitty. We see him as he logs onto his eHarmony profile and looks at the profile of a woman he admires. We also learn a great deal about the woman, Cheryl Melhoff, and the circumstances of her life, the fact that she has a child and a love/hate relationship with her sister. By looking at the internet lives of these two characters, we learn a great deal about Cheryl and Walter even before either of them has spoken a word.
Some of the humor of the film comes from the fact that Walter is a rather average, nondescript character trying to find some vitality in his drab existence. His world is a gray one (literally)—from his apartment complex to the color palette of his clothing—and the visual vocabulary of the film signals to the audience that Walter is trapped in a boring life, and takes a conservative approach to nearly all matters. He cannot even bring himself to talk to someone he works with, and resorts to pursuing her from a distance on an online dating service.
For all the drabness of his life, Walter has an intense and over-the-top imagination. On the train platform waiting to commute to work, Walter kicks into an epic fantasy of himself saving the dog of the woman he loves from a burning building. His heightened fantasy and imagination stem from the fear that he has never done anything noteworthy, that he is trapped in an unmemorable and forgettable existence, in which he never goes after the things that he wants.
The film often pivots between explorations of the fantastical and the more banal realities of life. Walter certainly switches between a colorful fantasy life and a world that seems to conspire against him, but other characters do as well. Cheryl has a similar tendency to switch between the heightened and the mundane, though she has an easier time integrating it into her social persona, and is humorous and witty. When she tells her coworker what she did over the weekend, she pretends that it was epic and memorable, before jokingly clarifying that she simply waited for a refrigerator repairman to fix her refrigerator.
Walter's life rapidly goes from drab and boring to exciting and high stakes when he receives a package from Sean O'Connell. O'Connell has told the magazine that negative 25 is the negative that should be on the front cover of the magazine, but inconveniently enough, negative 25 is missing. In order to preserve his position at the magazine, Walter must piece together where the negative is without letting his superiors know, lest it compromise his position at the company.