Twelve-year-old twins, Leon and DeeAnn, and their little brother, Caleb, are devastated by the mysterious disappearance of their beloved great-uncle Azer. The grieving siblings are determined to be content with cherishing his left-behind mementos. But one evening, those items summon an antique express train into Union Station, Washington DC.

The kids' decision to board strands them on a farm near West Chester, Pennsylvania, where they're thrust into the secret Underground Railroad system transporting escaping in 1854. Removed from the twenty-first century into a time without electricity, antibiotics, and cars, they struggle to adjust. Fitting in and hiding their time-travel secret, however, are not their most difficult tasks. They must drive a wagon with a false bottom to trick the slave catchers and run at midnight on dark streets in search of a doctor. Even worse, DeeAnn must walk in broad daylight on a road with escapees slaves.Through it all, they're determined to work against nineteenth-century racial prejudice.

The kids' worst fears materialize when the Time Line Express is not available for return to their own century in Alexandria, Virginia. Will they spend the rest of their lives stuck in a difficult and dangerous era, never to see their parents and friends again?