The Doctor takes Donna on a random journey and, with the billions of planets in the universe, just happen to land on a planet that produces Ood slaves.  They pose as people on tour of the plant and find that the red eye madness is starting to take hold of the Ood.  As more and more Ood fall to this "disease", and some even become rabid, the pair must dodge the human guards to figure out what is behind all of it.