Dangerous Liaisons: The Relationship Arc in Mystery Novels

Not all mystery novels include a relationship arc, particular in classic or traditional mysteries such as the ones by Agatha Christie and Sir Arthur Conan Doyle. In those novels, the detective is like a fixed point. Sherlock Holmes and Watson have a certain dynamic, but it doesn’t develop in any significant way across the stories. Watson accepts his role in this partnership. Hercule Poirot and Miss Marple are also static and do not get arcs. They get the job done. That’s why their clients hire them. And readers appreciate their consistency. They are the centre of order in the story.

This is one way the mystery genre historically differentiated itself from other genres. The focus is primarily on the external plot, and if there is a romance or other relationship subplot, it’s very secondary. Mostly the detective doesn’t change, and their interactions are strictly business.

Modern mystery novels, however, tend to incorporate a relationship arc for the detective in some form either to add more tension or to illustrate the emotional cost of the job. If mystery novels are fundamentally an attempt by the detective to impose order on a chaotic world, then messy relationship arcs become a great way to add more chaos.

First, what is a relationship arc?

Simply put, it’s the way a relationship changes over the course of the novel. Relationships, like characters, should be dynamic rather than flat. Things happen, rivalries appear, power struggles ensue, love develops or dies. Bonds are tested with betrayals, misunderstandings, competing goals—you name it, two people can get into it, and adding a relationship arc to your novel can go a long way to creating tension.

How does this relate to mysteries?

Because most mystery novels are told from the point of view of the detective (or amateur sleuth), any relationship arcs will implicate them directly. Let’s look at the relationships you’re most likely to focus on.

Detective + suspect: This arrangement can do a lot of heavy lifting for you. If a detective gets involved in some form with a suspect, it can potentially compromise the investigation. When a murder occurs and the detective is evaluating suspects, they won’t look as closely at a friend—and certainly not at someone they’re falling in love with.

Kinsey Millhone makes this mistake in A Is for Alibi. In fact, the suspect in question, Charlie Scorsoni, purposely manipulates her into a romantic relationship for that very reason. If she’s sleeping with him, she’s more likely to trust him—and more likely to cross him off her list of suspects.

Conversely, a detective who gets an immediate bad feeling about a suspect might become biased and blinded to other possibilities, as happens in Postmortem with Pete Marino and the suspect Matt Petersen, creating a nice red herring as well as lots of conflict with his colleague. Which brings us to…

Detective + colleague: This can be another great source of tension, especially if you create a rivalry that threatens to derail the investigation, as Patricia Cornwell does between Kay Scarpetta and Pete Marino. Detective versus police is a common source of rivalry, and in this case Scarpetta and Marino have very different approaches to solving a crime. They don’t (and can’t) succeed in catching the murderer until they finally have a reversal in their relationship and start working together.

The rivalry in the detective/colleague relationship can run the gamut from good cop/bad cop to expert/rookie, and of course romance can creep into the story as well, threatening to turn the investigation upside down and undermine the detective’s attempt at imposing order on the fictional world.

Detective + client: This is a key relationship in a mystery novel that requires a lot of trust to function. Consequently, the person who hires the detective can cause all sorts of trouble, especially if they hold back important information or give the detective a reason not to trust them. Ditto if it turns out they’re more implicated in the crime than they initially indicated.            

And it might not exactly be the client, as happens in Raymond Chandler’s The Big Sleep. In this case, it’s the client’s daughters who cause all the commotion for Philip Marlowe and make his job a lot harder. In fact, they turn out to be the main source of trouble.

Detective + victim: This can be a powerful relationship to work with because it might raise the stakes considerably if the detective has (or had) a personal relationship with the victim. Suddenly, solving the case becomes fraught with emotion and memory, and it might also mean your detective discovers things about the victim they didn’t know (and maybe didn’t want to know). Paging Gillian Flynn! Gone Girl manages to entangle detective, victim, and perp in one messy marriage (if we consider Nick an amateur sleuth).

A previous relationship with the victim also allows for the potential to beef up your detective’s backstory. Even if the detective doesn’t know the victim personally, it’s possible to create a link by giving the detective and victim something in common that creates more empathy and/or raises the stakes.

Detective + witness: This is a relationship that can be a good source of obstacles. A witness to a crime will be afraid and might even be getting pressured to keep quiet. The detective will have to earn their trust and promise them something they might not be able to deliver: safety. This relationship can be bolstered if the crime in question points to some backstory that the detective is struggling with. In other words, it might give them a personal investment in getting the witness to come forward.

In Jim Butcher’s Storm Front, the wizardly detective Dresden summons a faery for its testimony, which creates a nice combination of interrogation (the faery is a reluctant witness), danger (faeries are powerful and unpredictable), and world-building (goes without saying).

Detective + informant: This relationship is often morally complex. The detective might have to offer something important in order to get the information they want—and it could be something that compromises their code. Of course they’re getting something in return, so the relationship is transactional and built on mutual need, but one thing that might be shaky here is trust. And if the detective develops a fondness for the informant and the informant is threatened, you can develop a strong emotional subplot that shows the human side of the job.

In The Big Sleep, Chandler gives us Harry Jones, an informant who protects his source with a lie and pays for it with a whisky full of poison.

Detective + antagonist/villain: This is often a cat and mouse scenario where the detective might be closing in on the culprit while they in turn goad the detective or perhaps even threaten them. A certain respect for the other’s intelligence might even develop over time. In most mysteries, the reader doesn’t get the antagonist’s point of view (unlike in thrillers where we often do), so the detective’s gradual compilation of personality traits and details about the antagonist becomes like a puzzle that’s being solved in front of our eyes. This is one of the few relationships that Doyle develops over the course of his books—the rivalry of Holmes and Moriarty.  

Change is the important thing

How things begin in a relationship arc is not how they should end. Aim for a dynamic relationship that has ups and downs and progresses toward something. The shape is like a plot arc with an inciting incident, a problem that needs to be solved, a reversal in the middle that changes the relationship drastically, and then a movement toward the climax.

Cornwell’s arc with Kay Scarpetta and Pete Marino illustrates this movement well. Each professional stands for a specific approach to solving a crime. Marino is old school and relies on intuition, while Scarpetta is a forensic pathologist and therefore embraces a more scientific approach. Each thinks their way is best, which means they’re starting from a position of acrimony and rivalry—which will not make for a good partnership but will create good drama.

Indeed, for the first half of the novel they work at cross-purposes until there is a reversal in their relationship based on some unexpected common ground: they’re both underdogs in the system. When Marino realizes this, it softens his attitude toward Scarpetta, and they finally start working together. They develop a mutual appreciation for each other’s approaches, and the relationship comes full circle when Marino saves Scarpetta from becoming the serial killer’s next victim.

In conclusion

Readers have changed over the decades and while they still enjoy solving a good puzzle, they’re also looking for emotional connection in the story. Adding a relationship arc to your mystery novel can provide this component while also creating obstacles, complications, and reversals, all of which will increase tension and humanize your detective.

In a world where chaos must somehow be overcome, if there are ways you can add to the detective’s chaos and complicate their lives, do it!