Seeking help, they land on an unknown planet – and find a nightmare world where oppressed peasants toil for the Lords who live in the Tower, and where all learning is forbidden – a society in a state of decay.

What is the terrifying secret of the Three Who Rule? What monstrous creature stirs beneath the Tower, waking from its thousand-year sleep?

The Doctor discovers that the oldest and deadliest enemy of the Time Lords is about to spring into horrifying action.