“It was Friday—our Friday, the grunty, smelly one, who up until an hour ago was someone we thought wouldn’t know what “subterfuge” meant, let alone be able to pronounce it. He had changed. There seemed to be a much more upright bearing about him. Perhaps it was because he wasn’t dragging his feet when he walked, and he actually looked at us when he spoke. Despite this, he still seemed like a sad-teenager cliché: spots, long unkempt hair, and with clothes so baggy you could dress three people out of the material and still have enough to make some curtains.

“Why don’t you tell us what’s going on?” I asked.

“You wouldn’t understand.”

I fixed him with my best “Son, you are in so much trouble” look. “You’d be amazed what I can understand.”

“Okay,” he said, drawing a deep breath. “You’ve heard that the ChronoGuard is using time-travel technology now in the almost certain knowledge that it’s invented in the future?”

“I get the principle,” I replied somewhat guardedly, as I still had no idea how you could use something that had yet to be invented.

“As weird as it might seem,” explained Friday, “the principle is sound. Many things happen solely because of the curious human foible of a preconceived notion’s altering the outcome. More simply put: If we convince ourselves that something is possible, it becomes so. It’s called the Schrödinger Night Fever principle.”

“I don’t understand.”

“It’s simple. If you go to see Saturday Night Fever expecting it to be good, it’s a corker. However, if you go expecting it to be a crock of shit, it’s that, too. Thus Saturday Night Fever can exist in two mutually opposing states at the very same time, yet only by the weight of our expectations. From this principle we can deduce that any opposing states can be governed by human expectation—even, as in the case of retro-deficit-engineering, the present use of a future technology.”

“I think I understand that,” said Landen. “Does it work with any John Travolta movie?”

“Only the artistically ambiguous ones,” replied Friday, “such as Pulp Fiction and Face/Off. Battlefield Earth doesn’t work, because it’s a stinker no matter how much you think you’re going to like it, and Get Shorty doesn’t work either, because you’d be hard-pressed not to enjoy it, irrespective of any preconceived notions.”

“It’s a beautiful principle,” I said admiringly. “Yours?”

“Sadly not,” replied Friday with a smile. “Much as I’d like to claim it, the credit belongs to an intellect far superior to mine—Tuesday. Way to go, sis.”

Tuesday squirmed with joy at getting a compliment from her big brother, but still none of it made any real sense.

“So how does this relate to Mycroft and time travel?”