Since time immemorial, the Citadel of the Time Lords has been secure from all external threats thanks to the might of its founders. But what happens when the threats are internal?

This week, Paddy and Tricia watch as The Doctor betrays his own people to one of the deadliest forces in the universe. Only a small band of defenders lead by Leela and K9 are all that stands in the way of The Invasion of Time.