James and Jessica, the Two Jays, are on holiday in the Derbyshire Peak District in England, staying near Dakedale Manor, which has been completely destroyed in a fire. Did young Sam Stirling burn his family home down? Miss Parkin, the housekeeper, says he did, and she can prove it. Sam says he didn't, and he can’t prove it. But Sam has gone missing. James and Jessica believe the truth lies behind one of the old iron doors inside the disused railway tunnel.