 Together they set about the task of observing and investigating the sundry assortment of fellow lodgers, knowing that some are completely innocent while others harbour dark and treacherous secrets which threaten the very nation.From the grasping landlady Mrs. Dewar (Irene Handl) and the meek maid Elsie (Withers), through to fellow boarders including a blind man (Adam), a Boer War colonel and his wife apparently in retirement, a travelling salesman, a scatty old biddy and a merchant of Argentinian meat, all come under suspicion before the wily pair of sleuths manage to untangle the web of lies and false leads to reveal who in the household is or is not a traitor.Filming took place in Pinewood Studios.