The holidays go haywire for the Griffins when Peter raids a trailer park to retrive his misplaced family gifts while Lois loses it to holiday stress, going on a rampage and scaling Quahog's Christmas tree. Lois is downed by a tranquilising sniper while Stewie makes a hearwarming discovery. There is a Santa Claus - and he will deliver plutonium.
Christmas turns out to be, well, Christmas, compared to a Y2K New Year nuclear apocalypse that destroys everything execpt for Griffins, cockroaches and Twinkies. Undeterred by the end of civilisation, the family goes into a deranged survivalist mode while Stewie mutates - which causes him to change a lot more than his diapers.