Subplots can add to the texture of a mystery novel. They certainly make the characters more believable. For example, in Alexander McCall Smith's The Full Cupboard of Life, the main plot has Precious Ramotswe investigating four suitors for her client, Mma Holonga. Meanwhile, in an interesting subplot, her fiancé, Mr. J.L.B. Matekoni, proprietor of Tlokweng Road Speedy Motors, has discovered that his main competitor, First Class Motors, has been doing shoddy work on clients' cars and overcharging for that work. Mr. J.L.B. Matekoni's pride in being a mechanic and his dismay at a fellow mechanic's underhanded ways makes his character more well-rounded - more believable. Smith has the gift of letting this subplot play out without taking anything away from the main plot.
Some subplots involve relationships among the characters. For example, in John Alexander Graham's The Involvement of Arnold Wechsler, the main plot involves a kidnapping, stolen drugs and the murder of the president of Hewes University. The most interesting subplot is the relationship between Arnold Wechsler himself, who's a professor at the University, and his brother David, who may or may not be involved in the crimes. The two have always had a troubled relationship, and we see the resolution of that relationship as the novel moves on.
Colin Dexter has also woven subplots focused on relationships throughout his Inspector Morse novels. Some of them involve Morse himself. For instance, in The Daughters of Cain, a relationship develops between Morse and Ellie Smith, a prostitute he meets as he investigates the murder of Dr. Felix McClure. That relationship forms a fascinating subplot and at the end of the novel, after Ellie has disappeared, Morse is determined to find her again. In The Secret of Annex 3, the main plot has Morse investigating the death of a hotel guest at a New Year's Eve party. There are, however, fascinating subplots throughout the novel, many of them having to do with the relationships among the other guests.
Some authors tie the subplots to the larger, main plot. Dexter does this in several of his novels; The Dead of Jericho is an example. In that novel, Morse investigates the relationships among the residents of a small, middle-class neighborhood as he investigates the death of one of them. Those relationships form interesting subplots to the larger mystery. Agatha Christie, though used one of the most ingenious devices for tying plots together. In Dead Man's Folly, we see a plot-within-a-plot. Ariadne Oliver has been asked to create a Murder Hunt - a scavenger hunt for clues to a "murder." The competition is to take place at a fair to be held at Nasse House, a beautiful estate. Competitors search for these clues and are to use them to name the murderer, the motive and the weapon. The novel has as its background the subplot about the fair and competition. It's tied, though to the main plot, which is the murder of Marlene Tucker, a local teenager who's been enlisted to play the "victim" in the Murder Hunt. Christie brilliantly wraps the plot and subplot together, and that relationship between plots gives Hercule Poirot vital clues that he needs to solve the murder.
Christie brilliantly fleshes out The Mystery of the Blue Train with subplots, too. That novel centers around the strangling death of wealthy young Ruth Van Aldin Kettering. Throughout the story, we also follow a really interesting and engaging subplot about some precious jewels her father, Rufus Van Aldin, bought for her before her death. We follow the story of how he bought them, what happens to them and their ultimate fate. Like Dead Man's Folly, the subplot and plot of The Mystery of the Blue Train are related in a creative way. We also follow the subplot of the relationships between Katherine Grey, who is a passenger on the same train that carries Ruth Van Aldin Kettering to her death, and two suitors. Those subplots add interest, depth and personality to the story.
When subplots are subtly woven into the story, and don't detract from the main plot, they add depth and richness to a mystery. They make the characters more believable and they keep the reader's interest. They are a two-edged sword, though, so to speak. When the subplot becomes too important in the story, too scattered, or not believable, it detracts from the story. So as a writer, I'm careful about subplots. For instance, in Publish or Perish, a graduate student, Nick Merrill, dies suddenly one night. Merrill's been working on a larger university study of effective use of technology in classrooms, and his classroom observations form a subplot to the novel. But I don't spend a lot of time describing the Center for Effective Teaching, where the study is based. I don't want to distract the reader too much. For me, the threads of a story should lead ultimately to the same place - the resolution of the mystery. Subplots can help add some beautiful threads, but shouldn't go in the opposite direction.
What do you think? Do you like subplots? Do you find that they distract you? What are your favorite subplots?