The Green Hands Gang strikes again... Thieves have stolen valuable papers and seem to have escaped through locked doors and fastened windows!

How did they do it? Great Uncle Robert believes Snubby's fantastic explanation, but Roger and Diana just laugh at it. Then, as their clever detective work involves them further in the mystery, they begin to wonder...

Is there a connection with Rilloby Fair, where Barney is working? The children think so. But which of Barney's workmates is the thief? And where will he strike next?