Ridge and Hardcastle are sent on what seems a simple enough assignment, but stumble onto a major government cover up in an abandoned English manor.

Many years ago, Quist worked on the infamous Manhattan Project, which was instrumental in the designing of the atomic bomb codenamed Fat Man, used to obliterate the seaside port of Nagasaki in World War II. This involvement has continued to weigh heavily on Quist's conscience ever since. Hearing the phrase "Say knife," he recognises a potential threat - one that is tied into a break in at a plutonium base. A group of young student activists has stolen several radioactive fuel rods, and are planning to construct their very own atomic device.