An ineffectual science teacher William Lamb (Will Hay) is hired by a school normally located in middle England (St Michael's) recently transferred because of World War II evacuation policy to the remote Dunbain Castle on the Isle of Skye, Scotland.

Posing as (amongst many other things) an Old Etonian, Lamb settles down into his new surroundings and becomes acquainted with the various local Scottish traditions and legends that abound and strikes up a friendship with one of the other masters, Hilary Tisdaile (Claude Hulbert).

A Nazi spy ring proves to be behind the killings, and is defeated by a British agent hidden amongst the staff.

In one of the more memorable scenes Lamb is trapped inside a secret room with the ceiling slowly descending upon him.

Mrs Wigmore (who has proved to be the traitor) tries to shoot her way to escape but the bullets bounce off the suit of armour which Tisdaile is wearing.