The Doctor, Romana and K9 — and a young stowaway called Adric — are trapped in the alternative universe of E-Space. 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.