The blue crystal that the Doctor took from Metebelis III in a previous adventure is desperately sought by the Eight Legs, a race of mutated spiders, as the final element in their plan for universal domination. With help from an old mentor, the Doctor realises the only way to foil the plot is to make the ultimate sacrifice. 

n the English countryside, Mike Yates, discharged from UNIT and recuperating at a rural Buddhist meditation centre, is having a casual stroll when he hears a strange sound coming from the cellar. He secretly peers in and sees five men — Barnes, Land, Keaver, Moss and their leader Lupton — conducting some kind of unusual meditation. He sees a strange glow appear on the mandala they are all sitting around, and as he moves closer, he............