Hetty Wainthropp, a lady of a certain age, visits childhood friend Edith and discovers that her husband, Frank, has a son by a previous marriage. Hetty decides to turn amateur detective to trace him. This gives her a taste for detection, and she decides to set up a private detective agency.

A deaf-mute young man out bird-watching watches a couple necking in a car, but amusement turns to horror when he witnesses a murder.