Alien Heart

In the TARDIS, the Doctor and Nyssa stumble across a trail of ten destroyed worlds, all of them obliterated by means of some utterly monstrous but utterly unknown device. The planet Traxana would seem to be next in line to suffer the same fate. But when the TARDIS lands on an outpost on Traxana's moon, Nyssa is carried away by a tide of giant green arachnoids, leaving the Doctor behind...

And the coming menace is closer than he thinks.