MrPeterson(DavidTennant)isananxioussupplyteacherwhotakesajobatanunderachievingprimaryschool,StBernadette's.Hestrugglestocareforbothhisuninspiredpupils,andhisheavilypregnantwife(JoannaPage).Helivesintheshadowofhisoverachieving'goldenboy'identicaltwinbrother(alsoplayedbyDavidTennant),whoisateacherataposhprivateschool,St.Cuthbert'sCollege.[5]Thebrothers'rivalryintensifieswhentheybothentertheirclassesintothenational'SingaSongforChristmas'competition.MrShakespeare(JasonWatkins)andOakmorearebackaswell,hopingtowinthecompetition.MrPetersonendsuptakinghisclass(aswellasadonkey)onanunauthorisedschooltriptothecompetition'slocationatChristmasCastle,buttheybecomelost,andembarkonanepictripacrossthecountry,travellingbyanymeansnecessary.MrPetersonmustuseallhisresourcesanddowhateverhecantoguidehispupilstosafetyandreachChristmasCastle,iftheyaretowinthecompetition.