The TARDIS materialises on a deserted beach where the Doctor and his friends are captured by primitive warriors and taken to the fabled city of Atlantis.

While Ara tries to save Polly from being turned into a Fish Person and Ben and Jamie try to escape from the mines, the Doctor discovers the full extent of Zaroff's plans.

With Zaroff's plan nearing completion, the Doctor and Ben realise the only way to save the world is to destroy Atlantis.