by Robert Barr
[Starring] Stratford Johns as Det. Chief Supt. Barlow, Philip Brack as Det.-Insp. Cook, Howell Evans as Det. Con. Morgan
with John Barron as Asst. Chief Con. Gilbert

When three habitual criminals meet to study a diagram it's not likely that they are interested in central heating or plumbing... unless it is in some way connected with the planning of a crime. Barlow acquires a copy of such a diagram, but without a key to indicate what it represents, or what the crime is, or where it is going to take place. Gilbert believes he can provide that key.