Demiurge and his Evil Lords were conducting an investigation of the ruined Throne Room trying to make sense of the carvings on the Throne of Kings. After spending time speculating on the meaning, the ruined throne beings to glow and a strange monster resembling a Kagami mochi appears. Demiurge tries to use his mental magic on it however to the demons' shock is immune to his spell. The Floor Guardian deduces that the creature is a Summon and under the control of a powerful Summoner. He orders his demons to destroy the threat in Nazarick, which proves tricky as the monster is not an easy foe to defeat with. When the last blow is struck the monster begins to glow and the entire room in engulfed in light.
Momonga, who visiting the Round Table Room for a meeting, finds Albedo and Demiurge hanging there in with a kotatsu and is confused. He investigates and discovers that a mysterious New Year mood has spread through Nazarick. Disturbed by the lax behavior of his Floor Guardians, Momonga conducts his own investigation and summons several Death Knights and Skeletons to search for any clues to the source of this New Year's mood.
Shalltear Bloodfallen is recruited as it seems the New Year's mood does not affect undead beings. He accompanies her to the 6th Floor where they find the Mare Bello Fiore and Aura Bella Fiora dressed in New Year kimonos playing Hanetsuki. Though everything seems okay, a fissure appears and Chaos Beasts emerge along with a new monster that resembles a kadomatsu. Easily dispatching them with the help of the Floor Guardians, Momonga wonders what type of monster the kadomatsu was. A Death Knight reports that two of its agents have returned with no abnormalities, making Momonga sure that whatever is happening is only affecting the dungeon. Deciding to go to the first level alone, he greets the protagonist, Slimeko and Solution Epsilon that have returned. He nearly loses composure when Slimeko wishes him a Happy New Year as it shatters his belief that the New Year phenomena is the only epidemic to the tomb. The protagonist explains a strange New Year's mood was settling in the outside world. It appears whatever is happening is spreading and the group now see the seriousness of the situation. Faced with this Momonga decides to recall Sebas Tian. Later a strange tori is found on the 1st Floor. Having discovered it to be a Hatsumōde and the source of the New Year epidemic Momonga and his allies enter it and find themselves in a changed Nazarick where all the floors have transformed into desolate misty deserts. Solution scouts the area and so far despite the looming eerie feeling finds no danger.
Though he encounters Entoma Vasilissa Zeta, baking New Year's confections. Unsure if Entoma is compromised the group decides to greet her. Entoma welcomes them and explains that she was making mochi, which Momonga finds odd as she is not supposed to have a Cook class and surmises that the phenomena is responsible. After killing a few more enemies as they travel, Momonga believes that if they defeat whoever is responsible for the change to Nazarick, all will be returned. The group defeat the two and are allowed to pass where they meet with Yuri Alpha and Lupusregina Beta. When Hearing a sound similar to a gunshot the party follows it deeper into the misty dimension. Solution scouts ahead only to return to report she found CZ2I28 Delta acting odd in that she shooting a metal plate in the air. Momonga is left confused and decides to see for himself, where Shizu is doing as Solution reported. Momonga asks if she is targeting an enemy only for the Automaton to state she is playing with the wind and continues to shoot in the distance as it's fun. It's apparent that Shizu is affected in some way though different compared to the other NPCs, though Momonga guesses that her Automaton race could be a factor. The group then runs into a Kadomatsu Type Beast and swiftly defeat it. They are attacked by a mochi monster, which they kill and are surprised when Entoma gleefully eats the leftover mochi pieces. When questioned about what transpired Entoma believes that they were only collecting mochi, making Momonga ponder on the mental effect of the New Year's atmosphere as his influenced subordinates are not perceiving these new Chaos Beasts as threats. The group realizes that the effect of the phenomena seems harmless though the summoned monsters only appear to take action when interlopers interfere with the holiday atmosphere. Since they are unable to locate a transfer gate, they reason that something is purposely leading them to some goal. The group defeat the two and are allowed to pass where they meet with Yuri Alpha and Lupusregina Beta.
They come across a shrine where Cocytus and Narberal Gamma welcome them to the Takemikazuchi Shrine. They explain behind the shrine is a path leading to the 10th Floor. Momonga wishes to pass but the two to his surprise block his back insisting they duel them. Cocytus and Narberal are unable to ignore their directive and Momonga and the others have no choice but to fight as they are under some influence. The group defeat the two and are allowed to pass where they meet with Yuri Alpha and Lupusregina Beta. When Yuri is asked why she left her post, Yuri replies that it was a holiday. All in all Yuri and Lupusregina appear to be mildly affected by the New Year atmosphere. Though they invite him write poetry though they are suddenly attacked by Chaos Beasts. After dispatching the monster the groups continue to participate in the festivities. Solution decides to scout ahead once again, and Momonga suspects that the "final boss" is awaiting them on the 10th Floor. To be safe Momonga plans to send in the Skeletons to gauge the strength of the enemy.
In the Throne Room, the group find a bunch of Chaos Beasts by the throne. Momonga orders his summons to eliminate the intruders. When the last enemy falls, a mysterious being appears and proceeds to annihilate the skeleton party. Momonga observing the battle through his summons until their defeat analyzes the entity as a strong Chaos Beast and deduces that it is unable to leave the Throne Room. Planning an all-out attack, Momonga and his allies cross the Gate. The entity introduces himself as Shiramochi-ō, before Momonga starts by using [Reality Slash] but the being remains unfazed. Momonga notices the entity's lack of personal anger. He surmises that the White Advent King lacks an actual will and acts similar to a raid boss. That being said, the group begins to battle the entity, injuring it to a point where Shiramochi-ō is forced to use area attacks.
In the end, Shiramochi-ō is defeated and Nazarick is returned to normal. Returning to the Round Table Room Momonga finds Albedo and Demiurge back to their usual selves. They discuss the events that took place and debate whether it was an intentional attack and the connection to the New Year. Momonga postulates that due to the rules of the New World, it was probably some random phenomena that was summoned via the Throne of Kings. In the end. they may never know, though Momonga cannot help how uncannily the situation they experience was just like a game event. Demiurge promises to research the event. All in all the event allowed Nazarick to celebrate New Year's and was written of as a fun adventure.