It was only a minor incident, just a whisper of a possibility, but then the police had long since given up any hope of happening upon the obvious.

“We are looking for a catalyst,” Hooper explained to the assembled officers. “In chemistry, sometimes the tiniest element, if added to other compounds, can cause the most explosive results. That’s what we’re looking for: a tiny psychological catalyst.”

It had sounded good when Coleridge had said it to Hooper, and it sounded even better when Hooper showed off with it to his constables. Coleridge might have the lines, but Hooper felt that he knew how to deliver them.

The potential catalyst that Trisha had found was tiny indeed. It had not even been interesting enough for Peeping Tom to broadcast it, but Trisha found it interesting, and so did Hooper.

DAY NINE. 12.20 p.m.

Kelly, Jazz and David were in the hot tub together. As usual, David was talking.

“It’s interesting what you said yesterday about wanting to be an actress, Kelly. Because actually everybody in here is acting. You know that, don’t you? This house is a stage and all the men and women merely players.”

“Not true,” Jazz replied, with his customary abundance of self-confidence. “I’m being my true self, guy. What you see is what you get, because everything I got is too good to hide.”

“Oh, what nonsense. Nobody is ever truly themself.”

“And how do you know that, Mr Clever Arse Mind Games?”

“Because we don’t completely know ourselves.”

“That’s rubbish, that is.”

“Well, admit it, Jason.”

“Jazz.”

“Whatever. Haven’t you ever surprised yourself, spotted some new and different personal angle that you’ve never seen before?”

“Well, I once squatted over a mirror. That was a bit of a shock, I can tell you,” said Jazz, and Kelly laughed loudly, a big, brash, irritating laugh.

Irritating to David, anyway.

“I was staring straight up my arse, man,” Jazz continued, grinning broadly, “and even I was having trouble loving it!”

David was suddenly angry. He took himself very seriously and liked others to do the same.

“I can assure you, Jason, that we are all actors in life, presenting ourselves as we wish others to see us. That is why those of us who actually are actors, like myself, understand our world and the people in it more fully than ordinary folk do. We know the tricks, we read the signs. We recognize that we live in a world full of performers. Some of us are subtle, some are hams, but every one of us is acting. Seeing through your performance, Jazz, is my bread and butter.”

Jazz didn’t reply for a moment. “That’s bollocks,” he said finally, which was sadly well below his usual natural wit.

David smiled.

Then Kelly leaned forward and whispered something in David’s ear. It was hard to catch, but there was no doubt about what she said. What Kelly said to David was: “I know you.”

Then she leaned back against the side of the tub and looked straight into David’s eyes.

David returned her stare, his superior smirk undaunted. He seemed unruffled.

He was about to be ruffled. Very.

For Kelly leaned forward once more and whispered something else into David’s ear.

DAY THIRTY-THREE. 5.30 p.m.

This time neither Sergeant Hooper nor Trisha could quite catch what Kelly said. None of the officers working in the room could work it out at all.

It sounded something like “Far corgi in heaven.”

“That can’t be right, surely,” said Hooper.

“It would seem unlikely,” Trisha agreed.

Whatever it was that Kelly whispered, David had understood it and had not liked it.

There on the screen his expression clearly changed, subtly – he was too good an actor for his face to give much away – but his expression changed. Suddenly the smug, superior smile had disappeared.

He looked scared.

DAY THIRTY-FOUR. 9.00 a.m.

Hooper showed Coleridge the tape the following morning.

“Whatever ‘Far corgi in heaven’ means, sir, and that is certainly not quite what she said, it indicates to me that Kelly knew David before they entered the house.”

“It’s possible,” conceded the inspector.

“I reckon probable, sir,” said Hooper, running the tape once more. “When she says ‘I know you’ I thought at first she meant she knew him psychologically, because that’s what David was talking about.”

“Of course.”

“But then she says the other stuff, the corgi bit, and that’s clearly something that only David understands, some secret or experience from the outside world that they share.”

“No doubt about that, sergeant,” Coleridge agreed, “but it doesn’t necessarily mean they’d met. Kelly may have recognized something in David that enabled her to work something out about him.”

“I don’t count Kelly as the brightest apple in the barrel, sir. Working things out is not really her thing. I think they’d met.”

“Well, if they had then that is certainly a most significant discovery. Our whole catalyst theory is based on the presumption that they were all strangers. If two of them knew each other then that changes the dynamics across the whole group.”

For the first time the two detectives felt they might have a shred of a lead.

“So how do you read it, then, sergeant? Do you think that whatever Kelly recognized in David she recognized from the start?”

“Not unless she was as good an actress as she’d like to be. That first day was an absolute blank for her, I reckon. She just ran around shrieking, jumping in the pool and falling out of her top. Can’t say I noticed a single reflective moment. No, I think that whatever it was that made the penny drop for Kelly happened later. At some point David gave himself away, and Kelly spotted something about him that she recognized.”

“In that case I imagine it would have occurred not too long before she revealed her knowledge to David.”

“For sure. Kelly does not strike me as the sort of girl to keep a juicy thing like that to herself. She couldn’t wait to slap our Dave in the face with it, particularly after the way he put her down the previous day about her acting ambitions.”

“Well, if that’s correct, then whatever she saw she must have seen between the conversation around the pool and the conversation in the hot tub. What were they doing on the evening of day eight?”

“Tattoos!” said Hooper. “They were comparing tattoos! I’ve seen the tape.”

“Well, let’s take another look at it.”

By the time Hooper had reloaded the video tape, Trisha had joined them, and together they sat down to study the faces of Kelly and David as the group discussed tattoos.

Supper was over and with the exception of Woggle the housemates were all sitting about on the couches. They had just completed a small task set by Peeping Tom in which each housemate was loaned a pencil and paper and had to write down their predictions of who they thought would be left in the house at the end of week seven. They were also encouraged to jot down any other thoughts they might have about how things would pan out. All the pieces of paper were then put in a big brown envelope marked “Predictions”, which was solemnly sealed and placed at the back of the kitchen unit.

It was after that that the conversation turned to tattoos. They all had something to exhibit except Dervla and Jazz.

“I’m too black,” Jazz said, “besides which my skin is too beautiful to be improved.”

“I don’t have an explanation as to why I don’t have any tattoos,” said Dervla. “Except to say that it is extraordinary to me that these days when people talk about their tattoos it’s the people who don’t have them who have the explaining to do. Maybe that’s why I don’t want one.”