Synopsis: A human survey team arrives on an alien planet. So does the TARDIS. Both crews discover a beautiful world, with friendly humanoid inhabitants. As the Earth team watches the locals, though, they see what they believe to be an act of human sacrifice. One person willingly places himself in giant spider's web, apparently awaiting the day when he'll be the spider's meal.
Horrified, the Earth team begin killing all the spiders to make the planet safe. The Doctor probes a little deeper though, and discovers a humanoid apparently hatching from an egg. The Doctor rushes back to the Earth team, only to find out that the spiders have now all been killed. Sadly, in so doing, the humans have permanently destroyed a beautifully symbiotic ecosystem. The humanoid inhabitants of this world didn't "sacrifice" themselves to the spiders. They gave food to the spiders, who, in turn, provided cocoons for the humanoids, from which the humanoids would metamorphose into humanoid butterflied.
The Doctor leaves, saddened that he could not work out what was going on in time to save this fragile relationship.