The Ood are peaceful beings, under usual circumstances. They’re trained to serve humans, and they do so most efficiently. It’s their reason to exist.
Except now the Ood are turning against their masters. Something is warping their minds and turning harmless beings into killers.
It isn’t their fault, this is what humans have turned them into by enslaving them, cutting them off from the song they share with all their kind.
They’re starting to break free, but they need some help; the Doctor and Donna know just what to do.
The circle must be broken so the Ood can sing.