Doctor Who. Story no. 126, Mawdryn undead

Turlough may look like a normal pupil at a boys' boarding school, but he's actually an alien from another planet. And when the menacing Black Guardian asks him to kill the Doctor in return for a trip home, the boy quickly agrees. By coincidence, Turlough's maths teacher is an old friend of the Time Lord's. But why doesn't the Brigadier remember that? The Doctor, along with his friends Tegan and Nyssa, is about to find out.