During the blistering oppression of a heatwave the body of General Manager Frederick Allen is found murdered in his office. A coded lock on the staff entrance points the finger of guilt towards the staff, but as the employees of Bray, Holden & Co. come under the experienced scrutiny of Superintendent Roy Malden, the respectable outward appearance of the established Lichfield firm begins to unravel and the true nature of the appalling crime begins to surface.