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.

Actual causes of death and accurate timing of the death can be essential to a defense in a trial. Dr. Hardy appears as the expert witness for the defense and finds himself opposed by his former professor.

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 woman is found dead by the side of a railway line. Hardy is brought in to help the police. The woman has been murdered, and as they investigate, the police and Hardy realise that they have a dangerous psychopath loose in their area.

An old man dies. When his housekeeper returns some unused tablets to the chemist, suspicion clouds the issue. Dr. Hardy is brought in to conduct various tests and he questions the principals in the case.

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.

A safe is blown. The job has all the trademarks of a known criminal. Fleming pulls in two men in for the crime but forgets he needs concrete information to pin the crime on them both.

The Hardys are enjoying the peace of a summer evening. There is the noise of a shotgun. A man is brought to Dr. Jo's surgery with a non-fatal shotgun wound but John decides that this is a case for the police.

A drunken driver is responsible for the death of his brother-in-law and business partner and for the death of the driver of the second car involved in the accident. Hardy's concern for the facts leads to an exciting conclusion.