A couple of mall elves are outside for a cigarette break when they smell something unpleasant emanating from a nearby sewer grate. They go to investigate and find the decomposing corpse of a man wearing a Santa suit.
Meanwhile, Brennan is visiting her father in prison. Max asks Brennan what her plans are for Christmas and she tells him she plans to go to Peru to look at recently-discovered ancient remains. Max confides in Brennan that he would like to have another Christmas with family and children. Brennan reminds him that since he and Russ are both in jail, this scenario is unlikely to happen anytime in the foreseeable future. Her cell phone rings and she leaves to go investigate the Santa crime scene.
On the ride to the crime scene, Brennan tells Booth about her conversation with her father. Booth suggests she pull some strings to try and secure a conjugal trailer at the prison in order to spend Christmas with Max and Russ. Brennan rejects his suggestion because of her plans in Peru and because she does not wish to be a "string-puller". Booth confides in Brennan that he is not much looking forward to Christmas because his ex, Rebecca, and her new boyfriend are planning to take Booth's son, Parker, on a ski trip to Vermont for Christmas, so Booth will be unable to spend the holiday with his son like he had hoped.
Once they arrive, Brennan, Booth, and Cam investigate the crime scene and determine that the victim was washed down the sewer line from somewhere uphill and that rats and bacteria sped up decomp. Booth and Cam jokingly call the victim "Santa," and Cam points out that in addition to wearing a Santa suit, the victim also sported a white beard and was overweight.
The victim is brought back to the lab, and the team determines that he was murdered by a strong blow to the side of his head. Cam is able to determine time of death, and Hodgins confirms her findings, adding that the bugs he found on the victim indicate he was around some other food source at time of death. They also find further evidence that points to the victim being the "real" Santa: Hodgins discovers his suit is hand-tailored and there is evidence on his teeth that suggests he often smoked a clay pipe. Angela provides a sketch of the victim that looks just like traditional drawings of Santa Claus. Zack expresses disbelief at the endurance of the Santa myth, stating that Santa would have to make 822.6 visits per second to reach the home of every believer. Zack feels that children must be stupid to believe in the myth, but Cam reminds him that children are supposed to believe in the unbelievable, and that Santa is magic and can therefore defy the rules of science and math.
Brennan returns to the prison, this time to visit Russ. She runs Booth's idea of the conjugal trailer by Russ, and tells him she has already talked to his girlfriend Amy, who is willing to bring her two daughters down to celebrate Christmas with the Brennans. Russ refuses to take part because he does not want Amy's daughters knowing that he is in prison, as he had told them he was working overseas. When Brennan returns to her office, she finds Booth sleeping on her couch waiting for the "squint squad" to find out something useful. She tells him of Russ's reaction to the plan, and he urges her not to give up on making Christmas happen for her family.
On the forensics platform, Zack is showing Hodgins x-rays of the victim's sacrum that indicate he had a remodeled fracture from being kicked by a reindeer. As Hodgins shares this newest piece of evidence with Angela, Cam approaches. The FBI technicians were able to get a partial print of the victim's gloves and identified him as a mall Santa who is legally named Kristopher Kringle.
Brennan and Booth go to search Kringle's apartment and are amused to find that he lived directly above a toy store. His apartment is, unsurprisingly, filled with Christmas lights and decorations, his closet filled with Christmas-themed attire, and his fridge filled with milk and cookies. Booth discovers a hidden compartment full of cash in one of Kringle's bureau drawers, mostly small bills except for a group of $50 bills with sequential serial numbers.
Brennan and Booth question Kringle's landlord, who tells them that Kringle has been renting from him for six years, before which he lived at "The North Pole", according to his lease agreement. Brennan tries to convince Booth that Kringle is in fact the mythic figure of Santa Claus, but Booth refuses to believe it. The landlord also provides Brennan and Booth with the name of the temp agency that Kringle worked for. Booth asks if Kringle always paid his rent on time, and the landlord tells him that he did up until the last few months. Since Kringle was obviously not short on funds with his drawer full of cash, Booth presses to know why Kringle stopped paying his rent. The landlord reveals that Kringle gave him an idea for a toy that made him a lot of money. While Kringle never asked for a cut of the toy's profits, he did stop paying his rent once the toy became such a success.
On the drive back from Kringle's apartment, Brennan reveals that she is disappointed in Russ for lying to Amy's children. Booth sympathizes with Russ and reminds Brennan that he also lies to Parker, when he tells him that Santa is coming. Brennan believes the Santa myth is about blackmailing children into behaving, but Booth insists it is not about blackmail, but about everyone agreeing that children deserve to have a little magic in their lives. Brennan sarcastically suggests that she should come up with a lie to tell Amy's daughters so that they can come to Christmas without finding out that Russ is in prison, but Booth tells her that her sarcastic suggestion is actually a good idea. He suggests that they get Russ civilian clothes and tell the girls that he has flown in from overseas to spend Christmas with his family.
Brennan and Booth then visit the temp agency where Kringle worked and question his boss. Kringle's boss tells them Kringle was his "Numero Uno Santa" and got all the best gigs, but insists that this status would not have caused another Santa to get jealous enough to kill him. The partners are approached by another of the temp agency's Santas, Jeff Mantell, who asks if something bad happens to Kringle. Booth confirms that Kringle was murdered and asks if anyone has any information that might be helpful. Mantell tells Booth he only met Kringle a few weeks ago at a diner. Mantell was down on his luck and out of work, and Kringle secured him a job at the temp agency. The other Santas confirm this was a familiar story with Kringle and that he was always looking to help anyone he could. He was well-liked and didn't seem to have any enemies. Kringle's boss tells Brennan and Booth that Kringle was a Santa year-round, but would not work on Christmas Eve, as this was his "special night".
Back at the lab, Hodgins informs Cam that the maggots found on the victim's body also feasted on something Hodgins identifies as bird's nest soup, a rare Chinese delicacy, right around the time of death. Hodgins also found bird's nest soup on the seat of Kringle's pants. Hodgins determines that the only place the soup is sold locally is a restaurant in Chinatown.
Before heading to the restaurant, Booth drops Brennan off at his office to meet with Caroline Julian, the prosecutor in charge of her father's case, to ask her to provide a written recommendation so her father can have use of the conjugal trailer for Christmas. Caroline agrees to write the recommendation, but only on the condition that Brennan kisses Booth under the mistletoe, to satisfy Caroline's "puckish" side. However, Caroline tells Brennan that Christmas trees are not allowed at the prison, ever since a prisoner fashioned a shiv out of a star.
Booth and Hodgins search the alleyway behind the Chinese restaurant, and Hodgins finds the same maggots found on Kringle's body in the restaurant's dumpster. They also find the sewer grate where the body was most likely dumped and several cash-less wallets at the bottom of the dumpster. Hodgins poses the theory that Kringle was a pickpocket and was murdered by one his victims. Since the wallets contain IDs, Booth goes to track down their owners, leaving Hodgins stuck in the dumpster.
At the Royal Diner, Booth and Parker are talking at a table while Brennan is eating at the counter nearby. Booth tries to convince Parker that going to Vermont with his mom will be fun. To make Parker feel better, Booth tells him that he really likes Rebecca's boyfriend, Brent, and that he, Booth, will be spending Christmas with Brennan in Peru. Parker leaves to go to the bathroom and Brennan calls Booth out on all the lies she overheard him telling to his son. Booth reiterates that lying to children to make them happier is not wrong and is perfectly in keeping with the Christmas spirit.
At the FBI building, Booth questions one of the pickpocket victims while Brennan looks on from observation. The man tells Booth that he had $900 stolen off of him by a pickpocket dressed as Santa Claus. The man admits to tracking down the Santa pickpocket and punching him in the face, but insists he did not kill him. He also claims the confrontation happened in front of a big box store, rather than the alley behind the Chinese restaurant.
Later, Booth enters Brennan's office to tell her the serial numbers on the bills stolen from the pickpocket victim match the serial numbers of the $50 bills found in Kringle's drawer, suggesting that Kringle was indeed the Santa pickpocket. He finds Brennan hanging mistletoe from her office ceiling. Brennan explains Caroline's insistence that they kiss under the mistletoe. Booth is reluctant at first, but agrees when Brennan tells him it's the only way she can make Christmas for her family. Caroline enters and Brennan and Booth share a sweet kiss. Caroline tells Brennan the trailer is all arranged, and Brennan and Booth, clearly flustered, split up to return to the respective workspaces and continue their investigation.
Back at the diner, Brennan and Booth are dining with Sweets when Brennan tells Sweets she has a crisis. Booth believes she is referring to their mistletoe kiss, but Brennan insists that was nothing. Instead, she is upset about Booth's claim that lying to children around Christmas is not only acceptable but necessary for their happiness. Sweets agrees with Booth, explaining to Brennan that it is one's responsibility as an adult to preserve a child's sense of innocence and wonder for as long as possible.
Using this new perspective, Brennan visits her brother again and proposes that they lie to Amy's daughters as Booth had suggested. Russ reluctantly agrees to the plan, but tells Brennan she needs to come too, even if it means canceling her trip to Peru.
Meanwhile, Hodgins discovers traces of brass in Kringle's head wound and Zack is able to determine that the wound was caused by a circular brass object. Cam hypothesizes that the murder weapon was a brass bell, suggesting that Kringle was most likely killed by another Santa.
Brennan and Booth visit the temp agency again to test each of the Santas' bells for blood traces. Jeff Mantell tries to refuse having his bell tested, but his boss reminds him that under the warrant, he has no choice. Sure enough, his bell tests positive for blood. Booth begins to make an arrest, but Mantell cries out that the Santas switch bells all the time, as all the bells are identical and impossible to tell apart. Brennan and Booth try to figure out a way to tell which Santa is guilty. Since Kringle was in the habit of lending out money, Booth theorizes that the murderer was also the pickpocket and paid Kringle back using the stolen cash. When Kringle caught the pickpocket dumping stolen wallets, a confrontation must have ensued, during which Kringle got hit on the head with the bell. Brennan realizes that whoever murdered Kringle must have gotten bird's nest soup on his suit while rolling around with Kringle in the alley. She decides that she and Booth must sniff the seats of each Santa's pants in order to determine who the killer is. The seat of Mantell's pants smells of bird's nest soup, and he is arrested for Kringle's murder.
That night, Brennan decides to postpone her trip to Peru to spend Christmas with her family, and Parker runs away from his mom's house in order to spend Christmas with his dad. Booth calls Brennan to wish her a Merry Christmas and tells her to go to the window of the conjugal trailer and open the blinds. Outside the fence, Booth and Parker are standing beside a beautifully decorated Christmas tree, and Brennan tells her family that it looks like they got their tree after all.