The boys at St. Michael spend most their time gambling, betting and setting booby traps for their Head Master, Dr Benjamin Twist. A new Governor inspects the school and demands the Head Master's resignation. Dr Twist defends his educational methods and is given a chance to show what his boys can do in a public examination. He comes, accidentally, into possession of the French paper and the boys insists on his showing it to them.
They pass with honours and are invited to Paris to receive the congratulations of the French educational authorities. Trouble starts when the father of one of the boys escapes from Dartmoor. He and his gang steal the Mona Lisa from the Louvre with the inadvertent help of Dr Twist and the boys - will they manage to clear their names?