How to plot a mystery novel using Save the Cat

This article is part of a series about how you can plot a novel.

In a previous post, I reviewed what I knew about writing a mystery novel. It was more of a general overview of the elements needed. This post focuses on adapting the Save the Cat story structure to outline the plot of your mystery novel. This article summarizes each step and shows how that step could apply to a mystery novel. If you would like to read a more in-depth article about this type of story structure, please see my article on it here.

Spoiler Alert: The examples are from Agatha Christie’s Murder on the Orient Express.

Act I: Setting the Scene and Committing the Crime

Act I is all about introducing your detective, establishing a seemingly ordinary setting that hides a dark secret, and unleashing the initial conflict.

• 0-1% Opening Image: Set the mood and hint at the story’s themes. Show your detective’s everyday world before everything goes sideways, often hinting at underlying tension or their current boredom.
  Example: The bustling, yet rigidly structured world of the Orient Express, hinting at the confined space and diverse passengers that will soon become entangled in a murder.
• 1-10% The Setup: Introduce your protagonist, their routine, and any simmering conflicts. Give your detective a past failure or trauma that drives their relentless pursuit of justice, making them relatable.
  Example: Hercule Poirot, a renowned detective, is introduced on his journey, establishing his keen observation skills and his meticulous nature, which are central to his detective work.
• 10% The Catalyst: This is the big event that kicks off the story. The primary crime in the story happens. This is usually the discovery of a murder or major theft, immediately posing a central question for the story.
  Example: The brutal murder of Mr. Ratchett aboard the Orient Express, immediately establishes the central mystery that Poirot must solve within the confines of the snowbound train.
• 10-20% The Debate: Your detective hesitates and struggles internally before fully committing to the challenge. They weigh the personal cost of taking on the case, possibly clashing with their current life or past traumas.
  Example: Poirot initially hesitates to take on the case, preferring to enjoy his journey, but is eventually convinced by M. Bouc, the director of the Compagnie Internationale des Wagons-Lits.
• 20% Break into 2: The detective commits to investigating; there’s no turning back. Your detective is officially on the job, moving from observation to active investigation.
  Example: Poirot formally agrees to investigate Ratchett’s murder, moving from a passive observer to an active interrogator, beginning his interviews with the diverse passengers.

Act II: The Investigation and the Downfall

Act II is where the actual struggle unfolds. Tension builds as the bulk of the investigative work takes place.

• 22% The B Story: A subplot is introduced, often involving a supportive friend or ally. This subplot often helps to deepen the detective’s internal struggles or provides a sounding board for their thoughts, so they aren’t just talking to themselves.
  Example: The developing relationship between Poirot, Dr. Constantine, and M. Bouc, who assist him in the investigation, providing different perspectives and acting as a sounding board for his deductions.
• 20-50% Fun & Games: This is the juicy middle! Your detective explores the new world of the investigation, facing challenges, experiencing wins, and suffering losses. This is the primary investigation phase where the detective examines evidence, questions initial suspects with clever inquiries, and uncovers early clues. This is also where you “plant” subtle items or skills that will be needed later, avoiding amateurish plot holes.
  Example: Poirot interviews each of the twelve passengers and the train staff, meticulously gathering clues, discovering discrepancies in their alibis, and uncovering their hidden connections to the victim or to each other. This includes finding the charred note, the pipe cleaner, and the wagon-lit conductor’s uniform.
• 50% The Midpoint: A major turning point occurs, raising the stakes or revealing new information. Your detective might have a big win, like finding a crucial piece of evidence, or a crushing setback, such as a primary suspect having an ironclad alibi. This often leads to a false solution.
  Example: Poirot discovers that Ratchett was actually Cassetti, a notorious criminal who escaped justice after orchestrating the kidnapping and murder of a child, Daisy Armstrong. This revelation provides a strong motive but also complicates the immediate search for a single killer.
• 50-75% Bad Guys Close In: The antagonist gains momentum, and pressure mounts on your protagonist. The killer, whose goal is to avoid capture, uses tactics like framing others or spreading false clues and red herrings. The stakes keep escalating.
  Example: As Poirot delves deeper, he uncovers more inconsistencies and red herrings deliberately planted by the passengers, making it increasingly difficult to identify a single culprit. The alibis become more elaborate, and the passengers become more defensive.
• 75% All is Lost: This is rock bottom. Hope dwindles, and plans fail. Your detective realizes their theory is completely wrong, or they are personally targeted or even framed for a crime they didn’t commit, leaving them feeling utterly defeated.
  Example: Poirot is overwhelmed by the sheer number of contradictory clues and the seemingly impossible scenario where every suspect has an alibi or a plausible motive. He faces a moment of profound doubt, realizing a conventional solution seems impossible.
• 75-80% Dark Night of the Soul: Your protagonist reflects on their journey, flaws, and despair. They confront their past failures and internal fears, grappling with the impossible task of solving the case.
  Example: Poirot isolates himself, reflecting on the nature of justice and morality, and grappling with the moral implications of the crime and its potential solution. He considers the possibility that conventional justice might not be served.
• 80% Break into 3: A glimmer of hope appears; your protagonist finds renewed strength and devises a new plan. The detective has an epiphany, realizing the truth was hidden in plain sight, often by revisiting a small, overlooked clue from Act I. They now understand who the killer is or what their motive is.
  Example: Poirot has a sudden epiphany, realizing that the solution is not that one person killed Ratchett, but that all of them did, each striking a blow in a coordinated act of revenge for the Armstrong case.

Act III: The Truth and the Transformation

Act III delivers the climax and resolution. This is the high-stakes moment where the detective confronts the opponent and the puzzle is definitively solved.

• 80-99% Finale: This is the epic showdown and climax of the story. Your detective confronts the perpetrator. The killer makes a crucial mistake, forcing them into a desperate last effort. The resulting plot twist should create external, internal, and philosophical shifts, feeling surprising yet absolutely right. This is also where the subplot gets resolved.
  Example: Poirot presents two possible solutions: a lone killer who somehow escaped, or the collective, orchestrated murder by all the passengers. He dramatically reveals the truth of the latter, confronting the passengers with their shared guilt and exposing the intricate planning of their revenge. The subplot of the supportive friends culminates in their acceptance of Poirot’s unconventional solution.
• 99-100% Final Image: The story wraps up, reflecting your protagonist’s transformation and the resolution of the theme. Resolve the conflict and provide closure for the characters. Your detective achieves a self-revelation and returns changed. Aim for an ending that feels earned, not too obvious or confusingly ambiguous.
  Example: Poirot presents the “simpler” solution of a lone killer to the authorities, allowing the passengers to escape legal repercussions. This resolution highlights the moral ambiguity of justice and leaves Poirot with a sense of closure, having served a form of justice that transcends legal boundaries.

Why This Structure Works for Mysteries

Writing a mystery is like being a master illusionist: you lead your audience down a winding path of clues and suspects, meticulously hiding the truth in plain sight. The Save the Cat blueprint is flexible enough to allow for greater depth and exploration while ensuring that crucial beats, like the inciting crime and the climactic false solution reveal, occur at the points where readers expect maximum tension and pacing.

As I often emphasize when talking about plot structure, using a solid framework like the Save the Cat gives you a strong narrative foundation. This foundation allows your character development (like your detective overcoming their internal conflict) and your plot’s excitement to truly shine.