Once Mr Chitterwick had given his evidence, thus clarifying that the elderly lady's death was murder and not suicide, it appeared a straightforward case. He had seen something being put into the lady's coffee cup, after all. But then friends and relatives of the accused appeal to Mr Chitterwick, claiming him incapable of such a crime. As Mr Chitterwick investigates, doubts begin to surface, until more evidence arises to hint at a more complicated set of occurrences . . .

A terminally ill man decides to murder someone evil, but an innocent man is accused of the crime. The well-meaning killer tries to prove his guilt (aided by Ambrose Chitterwick), and hires a lawyer to prosecute himself.