Part ThreeSeries Episode Number: 642
World Premiere: Sat 20 Sep 1986 - 5:47pm BST [BBC One] (United Kingdom)
First Broadcast: Wed 10 Jun 2015 - 8:00pm EDT [Retro TV]
Running Time: 24 minutes 18 seconds
3.90 Million (UK)Appreciation Index: 70 (UK)
Chart Position: 98 () (UK)
With the black light converter destroyed, the Doctor fears for the planet's safety and needs to find the Secrets of the Sleepers and discover why they are there.