We had traversed the flagged corridor. Sir Cyril opened a narrow door at the end.

“Follow me,” he called out. “This passage is quite dark, but quite straight.”

It was not a passage; it was a tunnel. I followed the sound of his footsteps, my hands outstretched to feel a wall on either side. It seemed a long way, but suddenly we stepped into twilight. There was a flight of steps which we descended, and at the foot of the steps a mutilated commissionaire, ornamented with medals, on guard.

“Where is Monsieur Alresca?” Sir Cyril demanded.

“Behind the back-cloth, where he fell, sir,” answered the commissionaire, saluting.

I hurried after Sir