Mr Peterson (David Tennant) is an anxious supply teacher who takes a job at an underachieving primary school, St Bernadette's. He struggles to care for both his uninspired pupils, and his heavily pregnant wife (Joanna Page). He lives in the shadow of his overachieving 'golden boy' identical twin brother (also played by David Tennant), who is a teacher at a posh private school, St. Cuthbert's College. The brothers' rivalry intensifies when they both enter their classes into the national 'Sing a Song for Christmas' competition. Mr Shakespeare (Jason Watkins) and Oakmore are back as well, hoping to win the competition. Mr Peterson ends up taking his class (as well as a donkey) on an unauthorised school trip to the competition's location at Christmas Castle, but they become lost, and embark on an epic trip across the country, travelling by any means necessary. Mr Peterson must use all his resources and do whatever he can to guide his pupils to safety and reach Christmas Castle, if they are to win the competition.