In near-future Japan with ultra-low birthrates, children are treasured but monitored. Santa is banned as dangerous. One snowy December 25th, middle-schooler Kazushige Sanda, descendant of Santa, is attacked by classmate Fuyumura who wants him to find her missing friend, Ono. "Santa, please find my friend." To protect children from adults, Sanda decides to become Santa and fight the adults.
On a Dec 25 without Christmas, Sanda is attacked by classmate Shiori. Mistaking it for love, he awaits her confession, but hears something else.
Amaya learns Sanda is Santa, restrains him and Shiori to force a kiss. Santa refuses, unlocks sled power to escape. Then Headmaster Oshibu appears!
Sanda gains lie-detecting power. Shiori tries to use it to find Ono, but doubt overwhelms Sanda. Shiori then tells a self-harming lie.
At a funeral, Santa fights Yagiuda but fears his power. His resolve returns, shattering the roof and leaving him exposed before all.
Sanda vows never to love, but in a typhoon his message to fiancee Niko fails and he's drawn to her. The next day, Yagiuda arrives as a teacher.
Santa blocks Headmaster Oshibu from attacking adult Ono, but the enhanced 92-year-old overpowers him. Santa asks the kids for their belief to fight.
Amaya ends his friendship with Sanda to save him. Meanwhile, the scarred Oshibu, comforted by Yagiuda, turns to Class 10 students who killed adults.
Saved from Namatame by Shiori, Santa vows to harden his heart, but when Niko asks for a kiss, Sanda panics and almost transforms before her.
Tetsudome says Santa lacks grit. With her aid Sanda becomes dark Santa, but doubts himself. After meeting Namatame again, he vows not to fight kids.
At the Peak of Youth Ceremony, Niko steals Sanda's first kiss at twilight. Meanwhile Yagiuda faces Namatame alone, offering himself as decoy.
Namatame wrecks the ceremony. Eating yakisoba, she's met by Sanda in child form. His body starts breaking down as Ono's own changes grow.
Santa fights Oshibu before the students. The headmaster thrives on cheers, but Santa regains power by revealing his truth. The duel peaks on the roof.
