NOTE: This spoiler was submitted by Jeremy

The film opens in Bristol, England, on Christmas Eve. The real Santa Claus (David Harbour) is in a pub drinking the night away before he gets back to delivering presents. He speaks to a nearby mall Santa who is happy to do his job, but Old St. Nick has become jaded and disillusioned with the holiday, as people only care for gifts and are becoming more and more “naughty”. Just before leaving, Santa leaves a present for the bartender’s grandson, who is confused as to how he knows about him. She follows him to the rooftop to admonish him, thinking he’s just another drunk, only to look up at the sky and see Santa riding away in his sleigh, pulled by his reindeer. Just as the bartender admires the fact that Santa is real, he barfs over the side of the sleigh, which lands on her face.

In Connecticut, Jason Lightstone (Alex Hassell) is meeting his estranged wife Linda (Alexis Louder) and their daughter Trudy (Leah Brady). While the two parents try to make the holiday nice for Trudy, they are going through a rough patch. The three head to Jason’s family’s mansion, as the Lightstones are incredibly wealthy. His mother, Gertrude (Beverly D’Angelo) is an abrasive CEO of her company, while Jason’s sister Alva (Edi Patterson) is desperate for their mother to notice her since she thinks Jason is Gertrude’s favorite child. She is joined by her dim-witted actor boyfriend Morgan Steele (Cam Gigandet) and her obnoxious teenage son Bert (Alexander Elliot).

Santa goes through Christmas with extreme disdain, dropping off stuff like money in stockings and other useless gadgets and gizmos. He eats the cookies and milk left out for him but also helps himself to other peoples’ alcohol. When he gets to the Lightstone house, he uses their massage chair.

Jason and Linda decide to let Trudy open one gift early – a pair of walkie-talkies. When they leave her room, they overhear her speaking on it as if she were talking to Santa, saying that her one wish for Christmas is that her parents get back together. Jason and Linda have looks of regret on their faces.

A team of mercenaries posing as staff initiates their plan when one of the mercs, Gingerbread (Andre Eriksen), knocks out the head of security. The leader, Scrooge (John Leguizamo), kills the man at the front gate before entering the Lightstone house and rounding up his other mercs to hold the family hostage. According to Scrooge, Gertrude had hoarded $300 million that was meant to be distributed to the Middle East, and the money is in a vault somewhere in the house.

Santa overhears the gunfire and attempts to get out of there, but he is found by a goon called Tinsel (Phong Giang). He attempts to beat Santa down, but he rises and fights back. The gunfire from Tinsel’s weapon causes the reindeer to get spooked and fly off, leaving Santa stranded. In the ensuing struggle, Santa and Tinsel fall out a window, but while Santa is okay, Tinsel is impaled on an icicle. Since he doesn’t have enough Christmas magic to get him out of there, Santa goes to the front gate and finds the operator dead. Trudy then speaks on the walkie talkie, as Santa picked up the other one in an attempt to call for help. When she asks for his help, Santa decides to stick around.

Another merc, Frosty (Can Aydin), comes across Santa when he makes it back into the house. They engage in a more brutal fight, with Frosty attempting to stab Santa, but Santa is able to use his magic bag of gifts to defend himself (though he is unable to magically pull out a useful weapon). Santa grabs the star from the Christmas tree and jams it into Frosty’s eye, then plugs it into an outlet so that the star electrocutes Frosty and sets his head on fire. When Scrooge tries to check in with his mercs, Santa answers Frosty’s walkie and informs Scrooge that Frosty is dead, and the rest of them will be soon as well.

Scrooge sends his guys to search for Santa while also taunting the Lightstones. He has his henchwoman Candy Cane (Mitra Suri) torture Jason by breaking one of his fingers using a nutcracker, and then attempts to use a bigger nutcracker for its more literal purpose. Before they can hurt Jason further, Trudy steps in to yell at the villains, claiming Santa is there to help them, until Jason tells her there’s no such thing as Santa and that parents put Christmas gifts under trees and say they’re from him. Trudy runs away crying and hides in the attic while the goons go after her. She talks to Santa again on the walkie-talkie, and he proves he’s the real deal by recounting the various gifts she had asked for over the years, restoring her faith in the man. When she mentions the bad guys, Santa looks at his naughty list and sees all their names on there while reaffirming to Trudy that she has always been on the nice list.

Scrooge continues to mess with the family by having the adults give Gertrude her gifts. Morgan presents a business proposal to boost his career, which Gertrude obviously has zero interest in, while Alva gives her a framed picture of the day she was born, which Gertrude also shows little care for. They then grab Jason’s gift, which is a bottle of whiskey and a note that he doesn’t want Gertrude to see. She reads it but then says whatever is on it can stay between the two of them.

Santa finds Candy Cane and attempts to knock her out, only for a toy Santa to activate and blow his cover. She alerts Scrooge, who goes with Gingerbread to find Santa, who is under the attic. The villains knock him out and tie him up, demanding to know who he really is. Santa then drops the villains’ real names (Scrooge is Jimmy, Gingerbread is Bjorn, Candy Cane is Kyra) and the things they asked for when they were kids. Scrooge reveals his hatred for Christmas because his family couldn’t afford to celebrate it when his father got laid off, and he attempted to steal presents from a neighbor’s house and ended up scaring their grandfather, causing the man to fall down the stairs and break his neck. From the attic, Trudy creates a diversion by using styrofoam through the vents to create fake snow, briefly causing the villains to believe in Santa and giving him a chance to use his magic to shoot himself up the chimney to escape.

Morgan attempts to escape by jumping out the window. This happens just as Gertrude’s extraction team arrives to rescue the family. Unfortunately, Morgan learns they’re working with Scrooge after the leader, Commander Thorp (Mike Dopud), orders his men to execute him. Scrooge talks to Thorp and warns him about Santa, and Santa then sees more bad guys’ names pop up on the naughty list, meaning he has more work to do.

While Santa is plotting his next move, he talks to Trudy about who he used to be. Centuries earlier, he was a vicious Viking warrior called Nicamund The Red, and he used his mighty hammer Skullcrusher to demolish his enemies. While he has since tried to abandon his murderous instincts, Trudy encourages him to put those skills to good use. Just as the extraction team begins to storm the house, Santa finds a nearby sledgehammer in the basement and begins to pulverize the goons, and also uses a sharpened candy cane as a shiv against the villains.

Trudy sets up “Home Alone” inspired traps under the attic just as Gingerbread and Candy Cane go after her. Gingerbread steps through a broken ladder rung, causing his chin to be impaled on a nail. Candy Cane attempts to get ahead of him by climbing over him, and she manages to see most of Trudy’s traps coming, though the girl still hits her with a few bowling balls. One of them rolls out of the attic and hits Gingerbread, causing him to fall ass-first onto a bunch of nails. He pulls out a nail, only for one more bowling ball to fall and hit the nail into his skull, killing him. Candy Cane goes for Trudy until Santa arrives and bashes her brains in before saving Trudy.

Scrooge and Thorp go under the house to find that Gertrude’s vault has been emptied out. Enraged, they go to confront Gertrude and hold Linda at gunpoint until someone speaks up. This forces Jason to stand up and admit that he stole the money in an attempt to make off with Linda and Trudy for a better life and to distance himself from the rest of his family. That’s what he wrote on the note to Gertrude, hoping she would see it the next morning before realizing what he did. When the villains are called away, henchman Krampus (Brendan Fletcher) threatens to kill Linda, Alva, or Bert. The three argue over who should get shot, leading them to distract Krampus so Alva can throw wine into the fireplace behind him to stun him. The three then bludgeon Krampus until Alva sticks a fire poker in his neck. Santa then brings Trudy back to Linda.

Scrooge and Thorp take Gertrude with them and begin to make an escape. More goons show up, but the other Lightstones fight back. While Santa goes after Scrooge, Jason and Linda fight off the remaining goons in a Nativity display. After killing the last few goons, Jason and Linda kiss passionately, which Trudy happily notices.

Santa uses a snowmobile to chase after the villains, killing the remaining goons before knocking Thorp off his snowmobile and then going after Scrooge. The two engage in a brutal fight, with Scrooge using every weapon he has to pierce Santa. He then finds the naughty list and sees his name on there, finally convincing him that this really is Santa Claus. Scrooge says that killing him will mean the end of Christmas, but since Scrooge now believes, this gives Santa enough magic to go up a nearby chimney, pulling Scrooge with him and causing his body to get crushed on the way up, shooting chunks of Scrooge out the top. Just as Santa feels victorious, Thorp shoots him four times and prepares to execute him until Gertrude, who grabbed a nearby gun, puts one in Thorp’s head.

The other Lightstones arrive and try to help Santa. Jason burns some of the money to keep him warm, despite Alva’s protests. It appears as though Santa is dying until Trudy tells him she still believes in him and always will. Linda then says she believes because Santa saved their family, and Jason follows. Bert, Gertrude, and Alva all begrudgingly join in, allowing Santa to be revived.

Santa thanks the Lightstones, who return the gratitude, and he finds the reindeer returning with his sleigh, along with the original Skullcrusher, with a note from Mrs. Claus. Santa bids the Lightstones farewell and flies off into the night to continue his Christmas duties.

During the credits, Bert makes a TikTok video over a dead merc’s body to let his followers know that Santa really does exist.

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Santa Claus has become a jaded and cynical alcoholic over the last few centuries after having seen people get more and more "naughty", causing him to lose his Christmas spirit. He ends up at the home of the wealthy Lightstone family, which is being taken over by mercenaries led by a man calling himself "Mr. Scrooge". He is there to steal a fortune hoarded by the Lightstone matriarch Gertrude. Santa's reindeer leave him stranded there, and before he can make off on foot, he is called upon by the youngest Lightstone child, Trudy, who still believes in him and asks for his help. Santa then decides to deliver some seasons beatings.

Santa, who used to be a vicious Viking warrior, brutally kills Scrooge's goons, who begin to slowly see he is who he says he is. Making matters more complicated is that Gertrude's extraction team meant to rescue them are also working with Scrooge. The money turns out to already be gone, as Trudy's father Jason had stolen it to make off with Trudy and his estranged wife Linda, as well as distancing himself from his snobby family. The family manages to kill some of Scrooge's goons while Santa kills most of the extraction team.

Scrooge and Commander Thorp kidnap Gertrude, forcing Santa to chase them. Santa fights Scrooge and uses his Christmas magic to pull him up a chimney, causing his body to be crushed on the way up. Thorp shoots Santa, but Gertrude shoots and kills Thorp. Santa is dying, but the Lightstones now believe in him, so it's enough to revive him. Santa and the Lightstones thank each other as the reindeer return to take Santa back so he can finish his Christmas duties.