Walter, a selfish workaholic who seems to care about nothing other than making money, works at Greenway Publishing Company. He takes the books away from a nun, due to her not having money paying for them.
Later, Walter is asked to reprint "The Puppy and the Pigeon" due to its two missing pages, but Walter refuses, citing the printing costs. He is called on the intercom from Deb, saying that someone send him a Christmas gram, when it actually Buddy who had traveled day and night from the North Pole to find him. Buddy sang a little song of how he loves his father, which Walter counts as "weird", Buddy mentions Susan Wells, prompting the incredulous Walter to ask Deb to call the security to eject him from the building. At his apartment, Walter eats his dinner alone in his room where he is considering over the loss of Susan Wells.
Walter sees Buddy sleeping inside Gimbels and ignores Buddy's calls to him as he walks to the Empire State Building to work. Later, he receives a nightie, a photograph with him and Susan Wells and a Christmas card from Buddy which was handed in by the security who threw Buddy out again. Walter receives a call from Deb that the police are on the phone, Walter answers their call, and it turns out that Buddy called Walter to bail him out of prison after exposing the Gimbel's Santa. Walter reluctantly leaves work to bail Buddy out, and thinks Buddy is insane before taking him to the family's pediatrician Dr. Leonardo who does a DNA test on both Walter and Buddy, which conforms that Buddy is in fact Walter's biological son. Dr. Leonardo advises Walter that Buddy was developing a child like state and tells him to take Buddy home to meet his family.
Walter and Michael are obviously put off by Buddy's childish behavior, Walter, seeing Buddy insane, plans to throw him out, but Emily advises him to let him stay until he "recovers". Walter has to spend some of the night, doing all the childish things for Buddy, for example, tucking him in with his duvet. The next morning, Walter prepares to go to work, and rips down most of the decorations that got in his way in the living room. Walter advises Buddy to get rid of the costume, but Buddy declines, prompting Walter to ask him to "lose the tights", Buddy does so, accidentally freaking out Emily.
Walter gets a phone call from Buddy about a strange noise until he realizes it was the heater, Walter tells him to stop making phone calls to him. Later, Walter's boss, Fulton Greenway is furious over the published failure "The Puppy and the Pigeon", causing the company to be in financial trouble. Greenway organizes a book pitch on Christmas Eve, and demands Walter to be there, or face dismissal, and Walter agrees, not wanting to lose his job.
Later, Walter is advised by Emily to take Buddy to work with him. Walter works in his office as Buddy reads a children's book, and after Buddy disrupts a phone call, Walter persuades Buddy to help out in the mailroom, in order to keep Buddy out of the way. Desperate to save the company, Walter and his associates Morris and Eugene Dupree, place a phone call on the speaker mode with the best-selling children's book author Miles Finch, in order to hire him. But the phone call was interrupted by Deb on the intercom, telling Walter that a phone call is coming from the mailroom supervisor, due to Buddy throwing an outrageous dance party in the mailroom. Walter finally has Finch to come round to his office on Christmas Eve.
Finch arrives at the conference room and starts discussing ideas with Walter about the cover of the new book until Buddy interrupts the meeting, telling Walter about his love. Buddy mistakes Finch for a elf, due to his dwarfism, causing the short-statured author to attack Buddy, before storming out in fury, ignoring Walter's attempts of persuading Finch to stay. Now finally having enough, Walter loses his temper and harshly disowns Buddy, who leaves heartbroken as Walter is given disapproving looks from the employees, including Deb, Morris, and Eugene.
Walter phones Emily, telling her that he is working late on Christmas Eve. Walter now feels very remorseful for his yelling at Buddy, until his thoughts of shame are interrupted by Eugene and Morris, informing him that Finch left behind a notebook that is filled with brilliant ideas, Walter comes off his thoughts and excitedly tells them to get the storyboard ready for Greenway.
During a meeting with Greenway, Michael burst in, telling Walter that Buddy ran away after reading his apology note. Michael manages to convince Walter that to put his family above his job for the first time, and Walter tries to ask Greenway to reschedule the meeting. When Greenway callously refuses, Walter and Michael tell him "up yours" before leaving the conference room, prompting Greenway to fire Walter.
Walter and Michael go into Central Park, and find Buddy who is looking for a engine that broke off from the sleigh. Walter apologies to Buddy, telling him that Buddy has shown him the true meaning of Christmas, and truly and finally accepts Buddy as his son. Buddy takes them to meet Santa for the first time, Michael and Walter are convince that Santa is real. After seeing that the Central Park are on their way to get Santa, Buddy formulates a plan with Walter and Michael. Michael takes Santa's list in order to get the people in New York City to believe Santa as Walter takes Santa's coat and tries to stop the rangers, but to no avail.
As the rangers chase Santa, Jovie leads everyone to sing "Santa Claus is coming to town", Walter is mouthing the words, but Michael encourages to start singing earnestly, Walter eventually does so, finally restoring the Christmas Spirit in Santa's sleigh, allowing it to fly.
Next Christmas, Walter is no longer arrogant and selfish, but now becomes a kinder and caring person, and starts his own independent publishing company, starting with the best-selling book titled Elf, written and illustrated by Buddy himself.