When the Doctor discovers that a long-dead friend is still alive, he knows why his visit to the lonely Himalayan monastery has led to a struggle to save the Earth! 
 

But normally the Yeti, giant man-like creatures living in the remotest peaks of the Himalayas, are seen only very rarely and are notoriously timid. 

What is the explanation for their apparent transformation into ferocious brutes, monsters with glowing eyes and savage fangs who are spreading death and destruction in the isolated valley of peace? When the Doctor arrives at the monastery, his first visit for three hundred years, he expects to be welcomed with open arms. But because of the mysterious killings the reception that awaits him is anything but friendly ...