A young girl, Helen Green, and her family (with the exception of her brother, Rod, who is unable to step) trek across the long earth to form a new community, Reboot, on Earth West 101,754.
After stepping across "The Gap" - a universe around two million steps from the Datum where the earth no longer exists - Joshua and Lobsang encounter the threat that has caused the slow migration of Trolls away from the gap.