Hay stars as the principal of a one-pupil correspondence college who whilst visiting his pupil at the Ministry of International Commerce to try and obtain his fees for which the pupil (played by John Mills) has refused to pay because of the lack of decent material he his being sent becomes mistaken for a eminent economist Professor Davys. Hay tracks down the pupil who is working in a junior position and kicks up that much of a fuss that the pupil promises to get him a job in the department. With the confusion that arises over Hay's identity he is ushered into a room and is asked to give a live radio broadcast on the B.B.C on his knowledge of  South American economics and the virtue of International trading.

Obviously knowing nothing about economics Hay tries to bluff his way through the broadcast. On being asked a question on why it is important for a countries export to balance with  it's imports Hay answers 'what goes in must come out' Hay continues the broadcast with a load of nonsense about nuts being the chief export of Brazil and port being the main export of Portugal. When asked what happens when saturation point is reached Hay asks 'you mean when the people of both countries are saturated with port and nuts..aahh well there's a chance for England to jump in and increase her export of salts... from Epsom!'

The real Professor has been kidnapped by Nazi agents and their replacement arrives at the ministry for the radio broadcast and Hay is found out. He is chastised, but  re-united with his pupil who suspects what is going on and sets about to uncover the Nazi spy ring with Hay's help .Hay dons many a disguise to track down the ringleaders and poses as a Scotland Yard Inspector, a hotel cleaner, a railway ticket collector and finally a nursing sister. He finally tracks the nazi agents to the Claremont nursing home where the Professor  is being held prisoner. Comic capers ensue and our duo set about rescuing him to get him back to London to stop the signing of the pact between the South American Republics which as been altered by the German agents.

The climax of the film takes the form of a very lively car chase with the Professor tied to the back of Hay's getaway car in a wheelchair which culminates with them crashing through a roadblock protecting a unexploded bomb which then blows up leaving our trio in tatters. They make it just in time to the Ministry to prevent the signing and by way of reward Hay is given a job that he was promised which is 'in a very highly placed position.' It turns out to be the position of Fire Warden on the top of the Ministry building putting out small fires during air raids.

One critic remarked of this film:-"Am I right in believing that the most popular British film comedians were almost without exception first known on the halls? They have in any case brought with them to the cinema something of the music- hall manner; something which satifies the British taste for the hurly-burly rather than the wisecrack, for the boisterous rather than the sly, the comic catastrophe rather than the happy ending...Will Hay with his evasions, chases, absurd disguises and the inevitable debacle seems to me streets ahead of the rest. I begin to think Mr Hay is in the right tradition."
 

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