In search of a cure for a sickness that's so far claimed six billion lives, scientist Nyssa arrives at an abandoned gene-tech facility on the toxic planet Helheim. 'Hellhole', more like. Nyssa's not alone. The TARDIS has also been drawn to the Helheim base—and in its cobweb-coated corridors, she soon runs into the Doctor, Tegan and Turlough, her travelling companions of half a century past. But who, or what, has engineered this strange reunion? The Black Guardian, perhaps? The answer's here, in the dark. With the Cractids. In the cobwebs.