The father of three children is falsely imprisoned, accused of spying, and the three are set out to prove his innocence.

In doing so, they are obliged to move to a place near a railway station, where they meet an Old Gentlemen who rides the same train every day, and helps them in their quest.

They also befriend a Russian exile who has been persecuted by the Tsar for his writings in support of poor people, and the grandson of the Old Gentleman, who is badly injured in a railway tunnel.

This classic of young people's literature influenced many subsequent authors, and continues to be a vivid reading experienced to this day.