Jonah fidgeted in his seat, and his chair fidgeted right along with him. In another mood, Jonah would have been fascinated by this—how was the chair programmed? Did it have a computer chip making it squirm? But right now Jonah was too distracted. He was sitting in a sterile, nearly empty room, waiting to travel back in time to an unknown era and unknown dangers. So all he could do was fidget.

You’d think, with time travel, there wouldn’t have to be any waiting, he thought grumpily.

Beside him, his eleven-year-old sister, Katherine, bounced in her chair—making the chair bounce too—and chattered away to...