The Doctor, Romana and K9 continue in their search for the mysterious Key to Time.
They are drawn to Earth in the 1970's. The tracer leads them to an ancient stone circle, where rituals have been taking place. But when they arrive there seems to be no trace of what could be the Key...
This was the One Hundredth Doctor Who story, and the Fifteenth anniversary a few days after episode four was broadcast.
Join us on Wednesday 14th September, from 7pm BST (GMT) on Twitter to find out the secrets of the Stones!