This is the only episode of this seven part serial that still exists in the BBC archives.
This episode was thought to be lost until 1987, when it turned up at a car-boot sale and was returned to the BBC archives.
In the original script, the Doctor and Edward Waterfield were to travel back to Earth in the year 20,000 BC and retrieve a caveman named Og, from whom the Doctor is to deduce the essence of humanity. The Daleks' plan was to eradicate this quality from every generation of man, thereby eliminating Earth as a threat.
This episode takes place in London on July 20, 1966 and England in June 1866.