In the last film of the series, Hornleigh and Bingham join the Army to investigate scrounging of military supplies, but end up uncovering fifth columnists, who are using the Royal Mail in a novel way to pass along info.
Quite amusing film, with a fairly suspenseful spy plot tagging along with the comedy. Calvert makes a quite lovely femme fatale, and Huntley a nicely unctuous villain. The setpiece with Harker convincing a professor the school he is applying to is actually a reformatory for violent youths is priceless.
Note: Wally Patch is prominently credited, but nowhere to be seen. He must have been replaced by Noel Dainton, who, though wearing corporal's stripes, is carrying a swagger stick and addressed as "sergeant major" by Harker.