“Simple,” said Friday. “The obscenely complex technologies that the ChronoGuard uses to power up the time engines contravene one essential premise that is at the very core of science: that disorder will always stay the same or increase. More simply stated, you can put a pig in a machine to make a sausage, but you can’t put a sausage in a machine to make a pig. It’s the Second Law of Thermodynamics. One of the most rigid tenets of our understanding of the physical world. You can’t reverse the arrow of time to make something unhappen—whether it be unscrambling eggs or unmaking a historical event.”

“The recipe for unscrambled eggs,” I murmured, suddenly remembering a family dinner we had about the time of the Jane Eyre episode. “He was scribbling it on a napkin, and Polly made him stop. They had an argument—that’s how I remember it.”

“Right,” said Friday. “The recipe was actually an equation that showed how the Second Law of Thermodynamics could be modified to allow a reversibility of time’s arrow. That you could unbake a cake with almost breathless simplicity. The recipe for unscrambled eggs is at the heart of reversing the flow of time—without it, there is no time travel!”

“So,” I said slowly, “the whole of the ChronoGuard’s ability to move around in time rests on their getting hold of this recipe?”

“That’s about the tune of it, Mum.”

“So where is it?” asked Landen. “Logically, it must still exist, or the likelihood of time travel drops to zero. Since your future self just popped up twenty minutes ago to make veiled threats, the possibility remains that it will be discovered sometime before the End of Time—sometime in the next forty-eight hours.”

“Right,” said Friday, “and that’s what I’ve been doing with Polly for the past two weeks—trying to find where Mycroft put it. Once I’ve got the recipe, I can destroy it: The possibility of time travel drops to zero, and it’s good night, Vienna, for the ChronoGuard.”

“Why would you want that?”

“The less you know, Mum, the better.”

“They say you’re a dangerous historical fundamentalist,” I added cautiously. “A terrorist of time.”

“But they would say that, wouldn’t they? The Friday you met—he’s okay. He’s following orders, but he doesn’t know what I know. If he did, he’d be trying to destroy the recipe, the same as me. The Standard History Eventline is bullshit, and all they’re doing is trying to protect their temporal-phony-baloney jobs.”

“How do you know this?”