Connor stood at the pay phone in the lobby.  It was one of those new standing booths that has two receivers, one on either side, allowing two people to talk on the same line at once.  These booths had been installed in Tokyo years ago, and now were starting to show up all over Los Angeles.  Of course, Pacific Bell no longer was the principal provider of American public pay phones.  Japanese manufacturers had penetrated that market, too.  I watched Connor write down the phone number in his notebook.

"What are you doing?"

"We have two separate questions to answer tonight.  One is how the girl came to be killed on an office floor.  But we also need to find out who placed the original call, notifying us of the murder."

"And you think the call might have been placed from this phone?"

"Possibly."

He closed his notebook, and glanced at his watch.  "It's late.  We better get going."

"I think we're making a big mistake here."

"Why is that?" Connor asked.

"I don't know if we should leave the tapes in that security room.  What if somebody switches them while we're gone?"

"They've already been switched," Connor said.

"How do you know?"

"I gave up a perfectly good pen to find out," he said.  "Now come on."  He started walking toward the stairs leading down to the garage.  I followed him.

"You see," Connor said, "when Phillips first explained that simple system of rotation, it was immediately clear to me that there might have been a switch.  The question was how to prove it."

His voice echoed in the concrete stairwell.  Connor continued down, taking the steps two at a time.  I hurried to keep up.

Connor said, "If somebody switched the tapes, how would they go about it?  They would be working hastily, under pressure. They'd be terrified of making a mistake.  They certainly wouldn't want to leave any incriminating tapes behind.  So probably they'd switch an entire set, and replace it.  But replace it with what?  They can't just put in the next set.  Since there are only nine sets of tapes all together, it would be too easy for someone to notice that one set was missing, and the total was now eight.  There would be an obvious empty drawer.  No, they would have to replace the set they were taking away with an entirely new set.  Twenty brand-new tapes.  And that meant I ought to check the trash."

"That's why you threw your pen away?"

"Yes.  I didn't want Phillips to know what I was doing."

"And?"

"The trash was full of crumpled plastic wrappers.  The kind that new video tapes come wrapped in."

"I see."

"Once I knew the tapes had been replaced, the only remaining question was, which set?  So I played dumb, and looked in all the drawers.  You probably noticed that set C, the set Phillips removed when he came on duty, had slightly whiter labels than the other sets.  It was subtle, because the office has only been active two months, but you could tell."

"I see."  Somebody had come into the security room, taken out twenty fresh tapes, unwrapped them, written new labels, and popped them into the video machines, replacing the original tapes that had recorded the murder.

I said, "If you ask me, Phillips knows more about this than he was telling us."

"Maybe," Connor said, "but we have more important things to do.  Anyway, there's a limit to what he knows.  The murder was phoned in about eight-thirty.  Phillips arrived at quarter to nine.  So he never saw the murder.  We can assume the previous guard, Cole, did.  But by a quarter of nine, Cole was gone, and an unknown Japanese man was in the security room, closing up a briefcase."

"You think he's the one who switched the tapes?"