Ted Winters and Philo Birch were about to cross the street in Brownsville, Montana, on their way to observe the demonstration of Professor Valdmere’s Atomic Ray when a car drew to a stop beside them and a young girl, the only occupant, asked: Can you tell me the right road to Langdon Prairie? And then she slumped in the seat in a faint. Before the boys could take her to a nearby house, she revived, and showed them a blood-stained piece of paper on which was written Langdon Prairie, Dad.
Who is your father? Ted asked, and the girls astonished them by replying, Professor Valdmere. From that moment, the boys’ efforts to assist Peggy Valdmere took them into one dangerous situation after another, as together these three young people tried to solve the mystery of the Professor’s disappearance and preserve the secret of the atomic ray!