Footsteps were coming closer, and I plucked up all my courage, but I never saw who opened the door, because once again the strange feeling swept me off my feet, flung me through time and space, and spat me out on the other side.

I found myself back on the doormat outside our house again, jumped up, and looked around. Everything seemed the same as when I’d left just a little while ago to go and buy Great-aunt Maddy’s sherbet lemons. The buildings, the parked cars, even the rain.

The man in black at the entrance of number 18 was staring across the road at me.

“And you’re not the only one to be surprised,” I muttered.

How long had I been gone? Had the man in black seen me disappear at the corner of the street and then appear again on our doormat? If so, I bet he couldn’t believe his eyes. It served him right.

I rang the bell frantically. Mr. Bernard opened the door.

“In a hurry, are we?” he asked.

“Maybe not you, but I am!”

Mr. Bernard raised his eyebrows.

“’Scuse me, I forgot something important.” I made my way past him and ran upstairs two steps at a time.

Great-aunt Maddy looked up in surprise when I raced through the door. “I thought you’d already left, my love.”

Out of breath, I stared at the clock on the wall. It was now exactly twenty minutes since I had left the room.

“I’m glad you’re here, though. There’s something I forgot to tell you. They have the same sherbet lemons at Selfridges but sugar-free, and the packaging looks just the same. But don’t buy the sugar-free sherbet lemons, whatever you do, because those give me … well, diarrhea.”

“Aunt Maddy, why is everyone so sure that Charlotte has the gene?”

“Because … oh, ask me something simpler, can’t you?” Great-aunt Maddy was looking rather confused.

“Have they tested her blood? Couldn’t someone else have the same gene too?” My breathing was slowly calming down.

“Oh, Charlotte is definitely a gene carrier.”

“Because it’s been found in her DNA?”

“My little angel, you’re asking the wrong person. I was always useless at biology—in fact, I don’t even know what DNA is. I was no good at maths either. Anything about numbers and formulas goes straight in one ear and out the other. I can only tell you that Charlotte came into the world on the very day calculated for her hundreds of years ago.”

“So your date of birth decides whether you have this gene or not?” I bit my lower lip. Charlotte had been born on the seventh of October, and I’d been born on the eighth. We were only a single day apart.

“More like the other way around,” said Great-aunt Maddy. “The gene decides the time of the carrier’s birth. They’ve worked it all out.”

“Well, suppose they made a mistake?”

One day’s difference! It was that simple. Someone had mixed the dates up. It wasn’t Charlotte who had this wretched gene, it was me. Or maybe we both had it. Or else … I sat down on the stool.

Great-aunt Maddy shook her head. “They didn’t make a mistake, my little angel. If there’s one thing these people are really good at, you can bet it’s arithmetic.”

Who were “these people,” anyway?

“Anyone can make a mistake now and then,” I said.

Great-aunt Maddy laughed. “Not Sir Isaac Newton, I’m afraid.”

“Newton worked out the date of Charlotte’s birth?”

“My dear child, I understand your curiosity. When I was your age, I was just the same. But for one thing, it’s sometimes better not to know all the answers, and for another, I really, really would like my sherbet lemons.”

“None of this makes any sense,” I said.

“That’s only how it looks.” Great-aunt Maddy patted my hand. “Even if you’re no wiser now than you were before, this conversation must stay private. If your grandmother finds out all I’ve told you, she’ll be furious. And when she’s furious, she’s even worse than usual.”

“I won’t say anything, Aunt Maddy. And I’ll go and get your sherbet lemons right now.”

“You’re a good child.”

“Just one more question. How long after gene carriers have first traveled in time do they do it again?”

Great-aunt Maddy sighed.

“Please!” I said.

“I don’t think there are any rules,” said Great-aunt Maddy. “Every gene carrier is probably different. But none of them can fix the times for themselves. If the travel is uncontrolled, it can happen every day, sometimes several times a day. That’s why the chronograph is so important. With its help, as I understand, Charlotte won’t be flung around helplessly in time. She can be sent to times that aren’t very dangerous, where nothing can harm her. So don’t worry about your cousin.”

To be honest, I was much more worried about myself.

“Then when a gene carrier is in the past, how long has she been gone in the present?” I asked breathlessly. “And the second time a traveler goes back in time, could it be all the way to the dinosaurs, when there was nothing but swamps around here?”

My great-aunt cut me short. “That’s enough, Gwyneth. I’ve no more idea than you have!”

I got to my feet. “Thanks for answering my questions, anyway,” I said. “You’ve been a great help.”

“I don’t think so. I have a dreadfully guilty conscience. I really shouldn’t be satisfying your curiosity, particularly since I’m not supposed to know about any of it myself. In the old days, when I used to ask my brother—that’s your dear grandfather—about all these secrets, he always told me the same thing. The less you know, he said, the better for your health. Now, are you going to get me my sherbet lemons? And not the sugar-free kind, remember!”

Great-aunt Maddy waved as I left.

How could secrets be bad for anyone’s health? And how much had my grandfather known about it all?

* * *

“SIR ISAAC NEWTON?” repeated Lesley, baffled. “Wasn’t that the force of gravity guy?”

“That’s him, all right. But he also calculated the date of Charlotte’s birth.” I was standing in front of the yogurts in Selfridges Food Hall, holding my mobile to one ear with my right hand and covering the other ear with my left hand. “Only the crazy thing is that no one will believe he made a mistake. Who’d expect Newton to get his sums wrong? But he must have been wrong, Lesley. I was born one day after Charlotte, and I traveled back in time. Not her.”

“That’s more than mysterious. Oh, this stupid thing is taking forever to start up. Come on, will you?” Lesley shouted at her computer.

“Lesley, it was so—so weird! I almost spoke to one of my ancestors! Maybe that fat man on the painting in front of the secret door, Great-great-great-great-great-uncle Hugh, for instance. Well, if it was in his time and not some other period. They could have had me sent off to a loony bin.”

“I hate to think what could have happened to you,” said Lesley. “I still can’t get my mind around this! So much fuss made of Charlotte all these years, and now this happens! Look, you have to tell your mum right away. You’d better go straight back home. It could happen again any moment!”

“Scary, right?”

“Very. Okay, I’m online now. First off I’ll Google Newton. And you just go home! Any idea how long Selfridges has been there in Oxford Street? Could have been a deep pit in the old days, and you’d fall twelve yards down!”

“My grandmother will freak right out when she hears about this,” I said.

“Yes, and then there’s poor Charlotte … well, just think, all these years she’s had to give up everything, and now she gets nothing in return. Ah, here we are. Newton. Born 1643 in Woolsthorpe—where on earth is that?—died 1727 in London. Blah blah blah. Nothing about time travel here, just stuff about infinitesimal calculus—never heard of it, how about you? Transcendence of all spirals.… Quadratics, optics, sky mechanics, blah blah—ah, here we are, here’s the law of gravity.… Tell you what, that bit about transcendental spirals sounds kind of closest to time travel, don’t you think?”

“To be honest, no,” I said.

A couple standing next to me were discussing the yogurt variety they were going to buy at the tops of their voices.

“Are you by any chance still in Selfridges?” shouted Lesley, who had obviously overheard the yogurt orders. “Go home!”

“On my way,” I said, waving the yellow paper bag containing Great-aunt Maddy’s sherbet lemons in the direction of the exit. “But, Lesley, I can’t tell them this at home. They’ll think I’m crazy.”

Lesley spluttered down the phone. “Gwen! Any other family might well send you to the loony bin, but not yours! They’re always talking about time-travel genes and chronometers and instruction in mysteries.”

“It’s a chronograph,” I corrected her. “The thing runs on blood! Is that gross or what?”

“Chro—no—graph! Okay, I’ve Googled it.”

I made my way through the crowds in Oxford Street to the next traffic lights. “Aunt Glenda will say I’m just making it all up to look important and steal the show from Charlotte.”

“So? When you next travel back in time, at the very latest, she’s going to notice that there’s been a mistake.”

“And suppose I never travel back again? Suppose it was just the once?”

“You don’t believe that yourself, do you? Okay, here we are, a chronograph seems to be a perfectly normal wristwatch. You can get them by the ton on eBay, ten pounds and upward. Oh, damn … wait, I’ll Google Isaac Newton plus chronograph plus time travel plus blood.”

“Well?”

“Nothing that helps us. At least I don’t think so.” Lesley sighed. “I wish we’d looked all this up earlier. The first thing I’m going to do is find some books about it. Anything I can dig up on time travel. Where did I put that stupid library card? Where are you now?”

“Crossing Oxford Street, then turning down Duke Street.” Suddenly I had to giggle. “Why? Are you planning to come here and draw a chalk circle just in case our connection suddenly breaks? But now I’m wondering what good the silly chalk circle was supposed to do Charlotte.”

“Maybe they’d have sent that other time traveler after her—what was his name again?”

“Gideon de Villiers.”

“Cool name! I’ll Google it. Gideon de Villiers. How do you spell it?”

“How should I know? Back to the chalk circle—where would they have sent this Gideon guy? I mean, what period? Charlotte could have been anywhere. In any minute, any hour, any year, any century. Nope, the chalk circle makes no sense.”

Lesley screeched down my ear so loud that I almost dropped my mobile. “Gideon de Villiers. Got him!”

“Really?”