There's a god in the machine - and the god is insane.
In the self-contained Habitat on Dramos, things are getting out of control. Twenty million humans and aliens are at each other's throats, the lid barely held on by the Church of Adjudication, who through their OBERON systems wield absolute power. And we all know what absolute power does.
Other things have been corrupted too. People, human and alien alike, are changing - mutating into something that, if left unchecked, could consume their entire enclosed world.
Arriving in this disintegrating cosmopolitan society, Peri falls in with the charismatic leader of Human First, a movement dedicated to bringing order out of chaos. Meanwhile, the Sixth Doctor is powerless, imprisoned and put to the Inquisition by a church that really thinks its God is coming back.
If somebody doesn't do something, and do it soon, nobody's getting out alive.