Prologue
The Doctor had been invited to Earth by UNIT in June, 1992. All he had been told was that evidence had been discovered indicating the presence of extraterrestrials on Earth…five centuries ago.
In an underground cavern, the Doctor and Nita moved down a tunnel illuminated by mining lights. A scientist, male, late forties, led the way.
"Archeologists were down here excavating when they found some remarkable artifacts." He pulled back a tarp to reveal a collection of artifacts from the late 1500s…a broken pair of spectacles, a pocket watch, a pistol, a canteen, gunpowder and gunpowder case.
"And it's been determined that they date back to the late sixteenth century?" The Doctor studied them. "The vision aide is typical of the era… the weapon is a Wheellock Pistol invented by Leonardo da Vinci in about 1500."
The scientist nodded. "Look inside the watch."
Nita examined the watch and opened it. The time had stopped long ago. The face was cracked and broken. There was an engraving: 'To W.S, with love. 30 November, 1589.'
"What makes you think extraterrestrials are related to this discovery?"
"The archeologists were having a problem with their equipment, something in the cavern was interfering with it. It turned out to be the cavern itself. Do you have your sonic screwdriver, Doctor?"
The Doctor took out his sonic and took readings, reacting curiously. "The composition of the exposed rockface has been altered by exposure to triolic waves."
"Which you're not going to find on Earth, in either the sixteenth or twentieth centuries."
"Triolic waves?" asked Nita.
"The byproduct of an energy source employed by very few species because of its deleterious effects on living tissue."
"We've confirmed that no one has been in or out of this cavern in centuries, so we're left with a five-hundred-year-old mystery."
"I'm curious, there are far greater experts on Earth who could investigate your mystery. Why bring the Doctor all the way here?"
The scientist gave them a quick, grim glance. "As we continued our excavation, we found on other thing I haven't shown you yet." He led them across the chamber. "We've tried to leave everything as we found it." He pulled back another tarp to reveal something that stunned Nita and made the Doctor very curious.
The scientist studied their reactions to seeing the Doctor's sonic screwdriver broken in half, dusty and old.