At Fetch Priory, in the English countryside, Doctor Fendelman (Dennis Lill) and a group of fellow scientists are conducting experiments on a skull that was discovered in Kenya, which they believe is 12 million years old.
Fendelman is intent on making a picture of the person the skull belonged to, he uses a sonic time scan, which sets off a series of events.
Onboard the TARDIS, the Doctor and Leela are struggling to control the time machine, they trace back the source of the problem to Fendelman's sonic scan, and set the co-ordinates for Earth to try and shut it down.
Meanwhile back at the priory, the scan is causing the skull to glow and pulse, it locks on to Thea (Wanda Ventham), one of the scientists, and creates a psychic link with her. Outside the priory grounds, something otherworldly has manifested into being, and it attacks and kills a passing hiker.
Upon their arrival on Earth, the Doctor and Leela are soon separated. Leela ends up at the cottage of old Ma Tyler (Daphne Heard), an eccentric westcountry witch, while the Doctor has a near miss with the alien creature in the woods, which ends with one of Fendelman's hired security guards getting killed by the monster.
Ma Tyler comes across the monster, and manages to escape with her life, as a result of the shock of her experience she goes into a catatonic state, thankfully the Doctor arrives on the scene just in time and helps to bring her back from the brink.
When they get to the priory, the Doctor soon identifies a creature, known in Timelord mythology, as a Fendahleen, he finds the skull, and nearly gets taken over by it, but is saved by Leela.
Realising that the entire planet is at risk from the Fendahl, the Doctor and Leela set off in the TARDIS to find planet 5, the home world of the Fendahl, only to discover that it has been sealed off by the Timelords in a time loop.
Thea is being taken over by the Fendahl, add to this a spooky black magic coven, whose leader mistakenly believes that he can control the Fendahl, and you're in for a doozy of a story!
This could be considered the last of the stories from the gothic period in the late 1970s. Graham Williams had just taken over from Phillip Hinchcliffe as series producer, and this story was commissioned by outgoing script editor and Doctor Who luminary Robert Holmes, before the move initiated by the BBC hierarchy to give the programme a more light hearted style.
Tom and Louise are on good form, and, according to one of the interviews on the DVD they were getting along much better after a rather frosty start. The guest cast are very good, and the whole atmosphere of the story is really foreboding and spooky.
There is a cast commentary, a making of documentary, production subtitles, and an amusing easter egg.
And for a bit of added value, I can strongly suggest downloading "Pedantangle" episode #206 of Radio Free Skaro for a very entertaining fan commentary, you can download it here