1. The criminal must be introduced early on, and must not be a character whose thoughts the reader has followed.
2. No supernatural agencies.
3. No more than one secret room or passage, and these should only be found in appropriate buildings.
4. No undiscovered poisons, or appliances which need a long scientific explanation at the end.
5. No racist caricatures.*
6. The detective must not be helped by an accident or unaccountable intuition.
7. The detective must not commit the crime.
8. The detective must not find any clues which are not immediately revealed to the reader.
9. ‘The Watson’, i.e. the detective’s friend who is often the main point of view character, must be of slightly lower intelligence than the average reader, and must not conceal any thoughts which go through his mind.
10. Twins and doubles must not be introduced unless the reader has been prepared for them.– Ronald Knox, 1928
Most of these rules can also be applied to a film or TV show. Although viewers do not usually follow anyone’s thoughts in a film or television series, they do usually follow the point of view of a particular character or small number of characters. In Knives Out, for example, the point of view character is Marta. Although the structure looks at first like a Reverse Whodunnit in the style of Columbo, by the climax it actually turns out to be a Fair Play whodunnit, with a minor exception relating to Rule 9 (Marta, who functions as the Watson, is very intelligent; Rule 1 is followed as the villain is mentioned early on).
The point of the “rules” is that the whodunnit is intended to be a puzzle. The reader does not really want to be able to solve the puzzle, as they will generally enjoy being shocked or surprised by the solution, but they want to believe that it is, in theory, possible to solve the puzzle themselves. That is why one of the key rules is that the reader or viewer has access to all the same information as the detective at all times.
Glass Onion does not follow the rules of Fair Play, as it comprehensively breaks Rules 8, 9, and 10 (and flirts with breaking Rule 4). For the first half of the film, the detective has enormous amounts of knowledge the viewer does not, including knowledge of the actual murder that is being investigated, which the viewer is completely ignorant of until the midpoint twist. The audience does not even realize who the Watson is until then, and the plot is suddenly revealed to revolve around a twin the audience had no idea existed until that midpoint reveal.
For an audience conditioned to expect an Agatha Christie-inspired story, this may be mildly irritating. And this film has certainly taken a lot of inspiration from Christie and from some of the films based on her works, even more than the first Knives Out. The title card appearing over dark water in Glass Onion evokes the 1978 film of Christie’s Death on the Nile starring Peter Ustinov, and the following scene showing a group of rich, glamorous people gathering to take a boat to a private island is reminiscent of its follow-up, 1982’s Evil under the Sun. The music as the camera pans across gorgeous Greek beach resorts also cleverly evokes the scores for both the 1978 film and Kenneth Branagh’s 2022 adaptation of the same novel.
The plot of Glass Onion is full of tropes and tricks that appear in Christie’s books; an exotic location, a dead blackmailer, at least two murders and an attempted third, the murderer pretending to be the intended victim, characters posing as their own siblings, an announcement that a murder will take place, an eclectic group of people invited to a private island, and the detective laying a trap for the villain. One of the biggest influences is Christie’s Peril at End House, which features a fancy holiday location, people impersonating their own close relatives, the detective tricking everyone and surprising them all with a dramatic reveal, and the murderer pretending that they are the intended victim to cover up their murder of someone else.
Christie is known for mostly following the rules of Fair Play in her whodunnit stories, and Peril at End House, despite the similar twists, is a “Fair Play” story. In the ITV dramatization made in 1990, Poirot conceals a significant fact from his Watson, his friend Hastings, that he did not conceal in the novel, but even then it is not a clue as such that he hides from him. It is unlikely that many readers work out the solution to the story, but they do have access to all the same information as Poirot and Hastings throughout.
