“I expect you were so scared that you imagined things, Jenny,” said the Professor, yawning. “I think you’ll agree with me that the wall is far too steep for anyone to climb — and I’m pretty certain I’d have heard it if a ladder had been dragged about the courtyard!”

“Well, I’m sure I’m very sorry, sir,” said poor Jenny. “It’s a good thing we didn’t wake Tinker — though I’m surprised Mischief didn’t hear something and come running down the stairs.”

“But Mischief is surely with Tinker, camping out in the field!” said the Professor in surprise.

“No — Tinker and Mischief are back, sir. I found them asleep in bed — but not the others!” said Jenny. “Maybe Tinker has quarrelled with them. Funny that Mischief didn’t come running out to see what was up — he must have heard us!”

“Mischief is clever — but not clever enough to open Tinker’s bedroom door,” said the Professor, yawning again. “Good night, Jenny. Don’t worry. You’ll feel all right in the morning, and that will be that!”

The Professor went sleepily to his room. He looked out of the window down into the courtyard and then across at the tower, and smiled. Dear Jenny! She did rather let her imagination run away with her! As if anyone in the world could get up into that tower room without a ladder! And now could a long, long ladder be brought into that small courtyard without either being seen or heard? The Professor yawned once more and climbed into bed.

But someone had been in the tower room! Someone very clever, someone very light-fingered! What a shock for poor Professor Hayling next morning, when he crossed the courtyard, unlocked the bottom door of the tower — walked up the spiral stairway — unlocked the middle door, and went on up the stairway again — and finally unlocked the top door and opened it wide.

He stood and stared in horror. The place was upside down! All his papers were scattered everywhere. He crouched down at once to see if any were missing. Yes — quite a lot! But they seemed to have been taken quite haphazardly — a few pages from this note-book — a few pages from that — some letters he had written and left on his desk to post — and good gracious, the ink was spilt all over the place — and the little clock was gone from the mantelpiece. So Jenny was right — a thief had been about last night. A thief that could apparently get through three locked doors — or else could climb up a long, long ladder that he had put outside without being seen — and taken away again!

“I’ll have to ring the police,” he thought. “But I must say it’s a mystery! I wonder if Tinker heard anything in the night? No, he couldn’t have, or he would have run to fetch me. It’s a mystery — a real puzzle of a MYSTERY!”