Doctor Who - Audio Details

The Doctor and Sarah land on Earth to discover that it has been invaded by the shark-like Pescatons. The Pescatons' planet is dying and they want Earth as it's replacment. The Doctor has met them before, their leader the monsterous Zor once tried to force the Doctor into rescuing the Pescatons from their planet, but he refused.

While avoiding a Pescaton in the sewer Sarah realises that they are vunerable to high frequency sound. The Doctor then lures Zor into a trap and uses his piccolo to destroy him and the invasion collapses.