In December 2018, in Lowell, Massachusetts, widow Claire Pierce struggles to keep her family together around Christmas after her husband, Doug, who was a firefighter, died in a fire. Their son Teddy has lost much of his Christmas spirit, while their younger daughter Kate keeps it alive and continues to believe in Santa Claus. On Christmas Eve, Claire is called in to the hospital to fill in for a co-worker, leaving Teddy to look after Kate. While watching old family Christmas videos, Kate notices an arm appear out of the fireplace and deduces that it was Santa. Teddy agrees to help her prove it, on condition that she gives him the video she recorded of him stealing a car earlier that day.

They set up a tripwire and hidden camera, and fall asleep in the living room. After Kate awakens and sees Santa, she and Teddy follow him onto his sleigh to get a closer look.

Santa rides off with the kids in tow, but he and the reindeer are startled when Kate makes her presence known. In the commotion, he loses control of the sleigh, causing it to teleport and crash in Chicago, Illinois after the towline connecting the sleigh to the reindeer snapped off from excessive stress caused by the evasive maneuvers.

On the ground, Santa introduces himself and tells the kids he has to locate his reindeer, hat, and bag of presents, which were lost in the crash, in order to resume deliveries and prevent Christmas spirit from declining. They offer to help him to avoid being permanently on the naughty list.

After they get no help from a bar, they take a red Dodge Challenger that was stolen by the bartender. They locate the reindeer, but encounter police seeking the stolen car. Kate goes after the reindeer while Teddy and Santa distract the police with a car chase, which culminates in Santa’s arrest.

Teddy reunites with Kate, and together, they find Santa’s bag. Going inside the bag to find help, Kate is teleported to the North Pole, where she encounters Santa’s elves, who agree to help after discovering that she comes from a “long line of true believers.”

Meanwhile, a gang of thugs grab Teddy and the bag, taking them to their hideout. There, he is saved by Kate and the elves, who are then made aware of the situation.

At the police station, Santa explains his situation to Officer Poveda. When he reveals his knowledge of the officer’s wish to reconcile with his ex-wife, Lisa, who shares that wish, he is taken aback, but still refuses to believe him, and locks him in the holding cell.

As the night progresses, however, Poveda notices a higher than usual number of arrests for Christmas Eve, a predicament Santa had warned would happen if Christmas spirit continued to fall. In an effort to slow the decline, Santa gathers all the inmates to perform “Santa Claus Is Back in Town“, which works with all the officers except Poveda. After receiving a call from Lisa inviting him for coffee in the morning, Poveda is finally convinced and agrees to release Santa.

The elves complete the repair of Santa’s sleigh and give him a spare hat. Realizing that there is only one hour left to deliver presents, the kids offer their help, with Kate tossing presents to Santa and Teddy driving the sleigh. The plan is successful, with Santa completing his last delivery just before sunrise.

Afterwards, Santa takes the kids home before their mother returns. They ask him if they will ever see him again, to which he replies not if he can help it, but will if he ever needs help in the future. Santa gives Teddy his hat as a memento, showing that he did not really need it to move quickly or navigate tight spaces.

When Claire returns, they find the living room decorated like their father used to do it. Opening their gifts from Santa, Kate gets the skateboard she asked for, while Teddy gets a magic ornament. When he hangs it, he sees his dad magically appear in his reflection, and they both express pride in each other.

Back in the North Pole, Santa reunites with Mrs. Claus. She asks what movie they are going to watch, and he takes out Kate’s video of their adventure with a smile.