One day, Dak, Sera, and their class goes to the Smithsonian Museum. As Dak soaks in everything, Sera is bored with the history. Suddenly, an earthquake rocks the building, nearly causing a Viking longship to fall on Dak and Sera. After the disaster, Sera experiences a Remnant, a feeling that something different would have happened if time hadn't been changed. After she goes to Dak to share it, he allows her to enter his parents' lab. Sera completes the Infinity Ring, a device that allows time travel. However, their parents' first attempt at time traveling fails, getting them lost in the time stream. Soon after that, a dozen Hystorians disguised as SQ members storms Dak's parent's lab. When Brint and Mari, the Hystorians' current leaders, introduce Dak and Sera to the Hystorians, only Riq stands out. He is far younger than everyone else, with dark skin and eyes. He also knows sixteen languages and is 16 years old. Shortly after that, the SQ storms the compound and Dak, Sera, and Riq escape by using the Infinity Ring and decide to go back in time to fix the changes themselves.
Dak Smyth and Sera Froste are heroes. They fixed the Great Breaks in history, setting the world on its proper course at last. Or did they? Strange things are happening in the present, and it all points to more trouble in the past. But the time travelers have no idea why the SQ is so interested in a Chinese alchemist, the trial of Galileo, or a Russian dog named Laika. With the clock ticking and no Hystorians to help them, this time Dak and Sera may not be able to fix what has been broken. . . .