A red herring is literally a smoked herring fish. Smoking a herring turns its flesh red. Samuel Pepys of diary fame in England in the 1660s records that one day he ate red herring for breakfast whilst having his shoes unsuccessfully cobbled. Since red herring have a strong smoked fish odor, the metaphoric idiomatic meaning has come to be that of a misdirecting clue, a distraction in an argument or mystery that keeps a person diverted from the true meaning or true event.

As used in a murder mystery like Chrisite's The Mousetrap a red herring is a false clue that will direct the audience to suspect the wrong person as the culprit and villain. Some red herrings in the play are these. One is the dark shadow Mrs. Boyle throws over Christopher Wren when she complainingly insists his credentials be checked. Another is the late and unplanned arrival at the guest house of Mr. Paravicini after his car bogs down in a snow drift. Only villains arrive late and unexpectedly. Right?

MOLLIE. I'll see about your room. (She moves to the armchair C) I'm afraid it's rather a cold room because it faces north, but all the others are occupied.
PARAVICINI. You have several guests, then?
MOLLIE. There's Mrs Boyle and Major Metcalf and Miss Casewell and a young man called Christopher Wren--and now--you.
PARAVICINI. Yes--the unexpected guest. The guest that you did not invite. The guest who just arrived--from nowhere--out of the storm. It sounds quite dramatic, does it not? Who am I? You do not know. Where do I come from? You do not know. Me, I am the man of mystery. (He laughs)

Another red herring is Detective Sergeant Trotter's appearance, having traveled on skis through the snow storm, and announcement that a murderer is at large and expected to travel toward the guest house. Heroes are never villains, are they? Mrs. Boyle's murder and Trotter's gathering of suspects together are more red herrings, after all, Boyle's murderer would surely be someone whom she had injured, and all detective heroes gather suspects in the drawing room, don't they? Jessica Fletcher does, and Hercule Poirot does.