Bob was ruffling through his notes. “I left out a page,” he explained. “He told us he had friends working at some of the studios. Some of them liked to make gadgets like he did. They told him of a prop dragon that was to be destroyed to make room for other properties needed in storage. He saved them the trouble, went down and took it apart there himself. Then he had it hauled to his place in pieces and put it together later.”

Mr. Hitchcock frowned. “Did it have wheels?”

“No,” Bob said. “That was another bargain he picked up. He found an old abandoned chassis from a float at the Pasadena fairgrounds, left over from the Rose Bowl parade. They let him have it for towing it away. He put the dragon on that.”

“Hmmm. Clever,” Mr. Hitchcock said. “Now, how was it that Shelby knew about the big cave and the tunnel, whereas my friend Allen, who lived almost directly over them, did not?”

“Well, to begin with, Shelby knew about the existence of the tunnel from his days as an engineer for the City Planning Board. But he only found a way into the tunnel by accident.

“A landslide from an earthquake had covered the big cave many years before either he or Mr. Allen lived there,” Jupe continued. “Shelby was walking along the beach one day and saw a fissure in the rock wall. He dug in and discovered the cave and then the tunnel. He told the Morgans. They helped him build the fake wall inside. It was to fool people who accidentally found their way into the cave, and stop them from going farther into the tunnel.”

“I assume they helped him make the fake rocks outside the entrance, too?” Mr. Hitchcock said.

“Yes,” Jupe said. “That was interesting, and well thought out, too. They had to work from the inside of the cave and not attract attention. It was only when they had it all constructed that they could afford to clear the outside rubble away, at night, and insert their own rock covering.”

Mr. Hitchcock nodded. “The Morgan brothers — were they responsible for the collapsing staircase on your first adventure there?”

Pete interrupted. “They didn’t want anybody around who might spoil their scheme, so they weakened that staircase to scare people off the beach. They spotted us from their boat when we fell down it. And when we didn’t leave, they came out of the ocean and pointed their spear guns at us. They figured that should frighten us into not coming back.”

“I see,” Mr. Hitchcock said. “I believe you mentioned they had disappeared in the original cave you entered. Did you solve that mystery?”

Bob was back to his missing page of notes. “They went down the same pit I fell in. It wasn’t quicksand. Just a lot of mud and water. With their gear, they were able to work their way through to an underground passage that came out in the other cave near the tunnel. Like cave diving. It was an alternative way into the large cave during the day. They couldn’t take a chance of disturbing the big rocks outside too much and possibly attracting attention. Incidentally, after they ran out of the cave that last night, they never came back. I guess they were ashamed of being scared.”

“And good riddance, I might add,” Mr. Hitchcock said. “The thin, reedlike object Shelby blew into, that made no sound but opened and closed the fake cave wall. Am I to assume that was a sonic contrivance?”

jape nodded. “It opened and closed the prop rock opening outside, too. It had two varying high-frequency sounds. But it was Mr. Shelby’s undoing actually.”

“Indeed, young Jupiter!” Mr. Hitchcock exclaimed. “How was that?”

“It was his experimenting with the silent whistle, his sonic beam, that attracted all the dogs to him in the first place. As you know, sir, dogs can hear a higher frequency wavelength than humans. Mr. Allen’s setter ran to him the first night it was released from the kennel. He didn’t expect that because he thought Mr. Allen was still in Europe. That meant he had to move fast. The other neighbourhood dogs had already deserted their owners at night and run to Mr. Shelby’s sonic whistle. He couldn’t get rid of them, and had a lot of work to do, getting the dragon ready, the bank-vault opening drilled and the tunnel tracks cleared to the bank. Rather then destroy the dogs, as the Morgans wanted, he merely put them to sleep by adding tranquillizing agents to their food.”

Mr. Hitchcock reflected some more. “The dragon roared, you said. Was that your imagination running wild, lads?”

Bob shook his head. “No, sir. That roar and a lot of other things, like a windscreen opening in front were controlled by instruments on the dragon dashboard. Jupe was pressing all the buttons he could find in order to get it going.”

“Now, this Mr. Carter,” Alfred Hitchcock asked, “did he get safely out of the cave after being bowled over by the escaping dogs?”

“Yes,” Pete interrupted. “He was gone when we went back there to pick up the equipment we’d left behind.”

Mr. Hitchcock nodded. “And was he actually the living descendant of the Carter who started the tunnel and lost his fortune at Seaside?”

Jupe smiled. “Yes. But although he knew there was a tunnel, he never did find out exactly where. That’s why he knew about the first cave, and the boards leading into the next one. He was constantly snooping round there, and was more of a problem and worry to Shelby and the Morgans than we were. I suppose that’s why he always carried his shotgun with him, because he suspected something was going on.

“After the staircase near his house collapsed, he got suspicious again and went down to investigate. That’s when he nearly ran into Pete.

“Mr. Shelby told us the boards in the first cave had evidently been put in a long time ago by smugglers or pirates. He assumed they had built the moving rock. He found it by accident, just as we did. He added the newer plywood pieces himself when some of the old boards rotted. He was afraid somebody else would discover the moving rock and then the big cave and tunnel. He must have been saving the moving rock for a private emergency exit, because he never told the Morgan brothers about it.”

“And you helped Arthur Shelby return the gold bars to the bank vault?” Mr. Hitchcock asked.

“No,” Bob broke in. “He thanked us for the offer but said it was his own responsibility. He didn’t want to get us involved in any kind of criminal activity.

“He managed to get the gold bars back inside, and left them lying around carelessly as another kind of joke. Then he patched up the hole they’d made. I suppose the bank will eventually discover the tunnel underneath their vaults. But we haven’t told anybody else, not even Mr. Allen.”

Mr. Hitchcock nodded. “It is all certainly possible, given a man of Shelby’s considerable engineering talents. And it all came about because he knew the complete underground history of the city of Seaside.”

“Yes, sir,” Jupe said. “And the present-day history, too. So he knew exactly what banks were accessible from the tunnel all the time.”

“I see. One thing disturbs me. You allege my old friend Allen lied deliberately, saying he saw the dragon enter the cave, when it was impossible.”

“I’m sorry about that, sir,” Jupe said. “We found out later it was a simple mistake. He had been halfway down the steps that time, but had forgotten it in his distress at losing Red Rover. Is there anything else, sir?”

“No, boys. Though perhaps I should meet this Mr. Arthur Shelby. A man ingenious enough to scare you three lads is a man I could use. After all, you must remember, horror is my business, too.”

“Thank you, sir!” Jupiter cried, and Bob and Pete echoed the words. The First Investigator leaped to his feet. “We’d better get going. We’ve taken up enough of Mr. Hitchcock’s time.”

There was a flurry of happy boys, and then they had gone.

“Mmmm,” Mr. Hitchcock murmured to himself. “I wonder if I should borrow that ingenious dragon Mr. Shelby devised. Since I’ve just purchased that large bus-like trailer for my holidays, perhaps it would be a good idea if I learned how to double-declutch the dragon in the cave first, before I ventured forth on the Los Angeles motorways!”