[Scroll down if you haven't watched it yet]

Remember the slave-like Ood from The Impossible Planet/The Satan Pit? Well, it appears humanity's intergalactic empire enslaved them in the 22nd century.

That is, according to tonight's episode.

The Doctor and Donna arrived on the Ood-Sphere, after Doctor set the TARDIS controls to random. The first Ood they find is rabid and half dead - after killing his master - and lying in the snow. What each Ood are prone to mutter is "the circle must be broken". Which, in true Doctor Who fashion, has us all wondering until the end!

In the meantime, the episode consists of the following;
1. Humans insisting that the Ood are willing to be slaves (cue reminders of slavery throughout history!).
2. Ood silently rebelling, by killing their human colonisers with the (artificially inserted) translation ball.
3. Doctor flashing his sonic screwdriver.
4. Donna facing possible death, with words from her tongue.

The circle turns out to be a force keeping the collective Ood brain in place, and dulling the sound of their collective song. The song is the Ood's communication language, and it seems the human solution to keeping the Ood subservient, is to quieten their natural language. Kind of like slavery and colonialism really.

The Doctor leaves the Ood-Sphere with a message that the Ood will immortalise them in Ood myths, and also with a warning - "Your song will soon end." Whatever that means.