“Your family was of the pure blood, and Cornwall is old, so very old. It is associated with Wales in legends. Arthur was known there, and the Romans of Britain huddled within its borders when the axes of the Saxons swept them to limbo. Before the Romans there were others, many, many others, some of them bearing with them scraps of strange knowledge. You are going to make me very happy, Tregarth.” There was a pause as if inviting comment; when Simon did not answer, the other continued.

“I am about to introduce you to one of your native traditions, Colonel. A most interesting experiment. Ah, here we are!”

The car had stopped before the mouth of a dark alley.

Petronius opened the door.

“You now behold the single drawback of my establishment, Tregarth. This lane is too narrow to accommodate the car; we must walk.”

For a moment Simon stared up the black mouth, wondering if the doctor had brought him to some appointed slaughterhouse. Did Sammy wait here? But Petronius had snapped on a torch and was waving its beam ahead in invitation.

“Only a yard or two, I assure you. Just follow me.”

The alley was indeed a short one and they came out into an empty space between towering buildings. Squatting in a hollow ringed about by these giants was a small house.

“You see here an anachronism, Tregarth.” The doctor set a key in the door lock. “This is a late seventeenth century farmhouse in the heart of a twentieth century city. Because its title is in doubt, it exists, a very substantial ghost of the past to haunt the present. Enter please.”

Later, as he steamed in front of an open fire, a mixture his host had pressed upon him in his hand, Simon thought that Petronius’ description of a ghost house was very apt. It needed only a steeple crowned hat for the doctor’s head, a sword at his side, to complete the illusion that he had stepped from one era into another.

“Where do I go from here?” he asked.

Petronius prodded the fire with a poker. “You shall go at dawn, Colonel, free and clear, as I promise. As to where,” he smiled, “that we shall see.”

“Why wait until dawn?”

As if being forced into telling more than he wished, Petronius put down the poker and wiped his hands on a handkerchief before he faced his client squarely.

“Because only at dawn does your door open — the proper one for you. This is a story at which you may scoff, Tregarth, until you see the proof before your eyes. What do you know of menhirs?”

Simon felt absurdly pleased that he could supply an answer the other obviously did not expect.

“They were stones — set in circles by prehistoric men — Stonehenge.”

“Set up in circles, sometimes. But they had other uses also.” Petronius was all unsuppressed eagerness now, begging for serious attention from his listener.