Three bullets, a dead man, and £50,000 in insurance. Troubles arise when Dr. Hardy discovers the Unknown Factor.

Penny Goodwin shoots her husband in cold blood and there appears to be no motive. Why she has killed him is far from clear. When Hardy starts an investigation, he uncovers a most ingenious plot which even Fleming will not believe.

Miss Daley has given up working because her old boss is leaving. She finds out that he is still with the firm and becomes very ill. She goes to see Jo Hardy and tells her that she's convinced she has heart trouble.

A girl is assaulted in a park and the only witness is her young daughter. The police are certain they know who did it, but the girl cannot identify the man in an identity parade.

Mr. Toller is a who has no time for the weak or the idle, without humour. Jo Hardy is treating him, he is arrogant and she patiently tries to diagnose what is causing his illness, which becomes more and more serious.

A man falls overboard from a ship near Hull and washed ashore dead. Injuries sustained in the water means he is unrecognizable. The most careful planning can go awry and only an expert can interpret the facts with which he is presented.