Something haunts the peak of Ben MacDui. Something with heavy footsteps, striking terror in the hearts of those who sense it. With climbers going missing, retired Brigadier Lethbridge-Stewart visits Scotland to investigate. But when some old friends join his ascent, he worries that they will make things even more dangerous. As the snows blow in, and mists surround them, the Doctor, Ace and the Brigadier will face the Grey Man of the Mountain...