Synopsis: Altrouge Brunestud-Emiya has decided to host this year's Christmas party, and invites everyone. Even that other clan, the Tohnos. Why? Because she wants to show up her sister Arcueid (and win a candy bet). And also to get Shirou and Shiki to stop trying to stab each other to death every time either man is within a hundred meters of the other. Meanwhile, four heroes (really, four people who don't want to drown in the inevitably-oncoming onslaught of insanity) try to break free of the trap.
Eleventh Hour Ranger: An old friend of both Shiki and Shirou, Chuck Norris shows up when everything's gone to Hell and offers to help fix it all. Apparently, even he can't overcome the insanity and ends up brawling with Santa Claus in Detroit.
Meanwhile Scene: Several. Mainly because there are so many people attending.
Personality Swap: Due to a pranks gone wrong, many of the Clan's personality were reversed.
Putting the Band Back Together: AU!Illya blackmails Eiko, DA!Haru, and BK!Kiri to form the !A Us. Subverted in that they weren't actually together to begin with, but all had played instruments at one point.
She Is All Grown Up: Shirou gets this when he sees Alter!Illya for the first time. Subverted in that she's from a different reality...with highly disturbing implications.
Shotacon: A personality shifting potion causes Aoko to temporarily regress to this.
They Killed Kenny Again: Like most of the Nasuverse Lancer gets this shoved upon him for the duration of the game show. It gets to the point where Illya and Zelretch are summoning him just to see how many ways they can make him suffer.
Thoroughly Mistaken Identity: Miya mistakes Tamaki for her son Takeshi even they look nothing alike. It's made worse by the fact that at that moment she's trying to kill Takeshi for having his first time with his ashikabi Haruhi Fujioka.