Young Imogen Burnhope and her maid Rhoda board a non-stop train to Oxford to visit her Aunt Cassandra, who waits on the platform at Oxford station to greet them. Only they never arrive. All the passengers alight but the two women are nowhere to be seen. The train is searched and the coachman swears he saw them board onto first class, but they seem to have vanished into thin air. When he learns his daughter is missing, Sir Marcus Burnhope contacts Scotland Yard for help and Inspector Colbeck and Sergeant Leeming are assigned to the case. Is it a simple case of a runaway? Or is there a larger, more sinister conspiracy at work? The Railway Detective must unravel the mystifying web of their disappearance before Imogen and Thoda vanish into oblivion for good.