A newly exiled Doctor accompanies Liz Shaw and the Brigadier to a symposium held in an imposing Scottish castle. Professor MacLeod and her son have developed a new treatment for memory loss and a possible cure for dementia. As the Doctor investigates sightings of strange mythical creatures, and ghosts are unleashed in the castle’s corridors, Liz and the Brigadier search for a connection. Just what is happening to the energy produced by the MacLeods’ revolutionary treatment? And what is the strange creature that hides in the castle grounds?