Mr. Satterthwaite is an upper-class gentleman of advanced years, a keen observer of life albeit mostly from the sidelines. Mr. Quin is a (seemingly) young man with a knack of turning up mysteriously just when something is about to happen and saying just the right thing or asking just the right question that will inspire Mr. Satterthwaite to solve the mystery.
It is frequently hinted that Mr. Quin may be supernatural in some respect; his sudden appearances and disappearances, his exquisite timing, and the frequent impression that he knows more than he's letting on, suggest it without ever providing solid evidence. There's also a recurring thing where a trick of the light will momentarily make him appear to be wearing brightly-coloured motley, or a trick of shadow will make him appear to be wearing a mask; these, along with his name, connect him to the magical central character of the harlequinade.
Christie was using a symbol that was better known in her time than now, which is why Mr. Quin always shows up just when tragedy in the form of murder or suicide is about to strike—especially when it's about to strike lovers. The harlequin in the Commedia dell'Arte is both likable and romantic. But in other tales, the harlequin is more sinister—a trickster who makes his own rules and who may bring chaos or death. Quin combines the two images, as he's both a likable romantic and someone who quite enjoys throwing a spanner named Mr. Satterthwaite into the works now and then. And judging by the frequency with which Quin knows when someone is dying or about to die, it's very likely that he's either an agent of death or is Death itself. Christie's words support that; in her autobiography, Agatha Christie listed the Harley Quin stories as her favorites and describes Harley Quin as "a friend of lovers and connected with death." Christie dedicated The Mysterious Mr. Quin to "Harlequin the invisible"—which doesn't make much sense if she was talking about a character noted for his sudden and unexplained appearances but does make sense if she was referring to an entity by a different name.
Quin and Satterthwaite appear together in two stories that were not collected in The Mysterious Mr. Quin. "At the Crossroads", the fifth story in original magazine publication order, was omitted from the collection; it's one of the weaker series entries, and Christie had already scavenged bits of it for the novel The Murder at the Vicarage. (It eventually made it into one of the miscellaneous Christie collections under the title "The Love Detectives".) "The Harlequin Tea Set" was written years later to round off the series. In the interval between The Mysterious Mr. Quin and "The Harlequin Tea Set", Mr. Satterthwaite made two appearances with Hercule Poirot, a cameo in "Dead Man's Mirror" and a full team-up in Three Act Tragedy.
The omnibus The Complete Quin & Satterthwaite, Love Detectives collects all the stories in which Quin or Satterthwaite appear.
The 1928 movie The Passing of Mr. Quin, the first film adaptation of any of Agatha Christie's works, is loosely based on the first Quin and Satterthwaite story but leaves out Satterthwaite entirely and retains Mr. Quin In Name Only. Three Act Tragedy has been adapted several times, always without Satterthwaite.
- "The Coming of Mr. Quin"
- "The Shadow on the Glass"
- "At the 'Bells and Motley'"
- "The Sign in the Sky"
- "At the Crossroads"
- "The Soul of the Croupier"
- "The Man from the Sea"
- "The Voice in the Dark"
- "The Face of Helen"
- "The Dead Harlequin"
- "The Bird with the Broken Wing"
- "The World's End"
- "Harlequin's Lane"
- "The Harlequin Tea Set"
These stories provide examples of:
- And the Adventure Continues: "The Harlequin Tea Set" ends with Quin assuring Satterthwaite that they'll meet again.
- Asshole Victim: Barbara Stranleigh in "The Voice in the Dark" is described as "beautiful, unscrupulous, completely callous, interested solely in herself", and turns out to have given her murderer good reason for wanting revenge.
- Better Manhandle the Murder Weapon: How the Obvious Suspect becomes the Obvious Suspect in "The Shadow on the Glass".
- Broken Bird: Mabelle Clydesley, the title character of "The Bird with the Broken Wing". She is described as having an ethereal tragic quality to her, she comes from an ill-fated family, she wears clothing that resembles bird-feathers, and she ends up a murder victim due to an insane attempt to grab attention that had nothing to do with her. Mr. Quin represents her in the story as a blue bird statue.
- Character Overlap:
- Mr. Satterthwaite's appearances in the Hercule Poirot series.
- A policeman in "At the Crossroads" also appears in The Secret of Chimneys.
- Comic-Book Time: Mr. Satterthwaite has been described as either 62 or 69 years old.
- The Ditz: A starlet had a young man condemned to two years in prison for stealing her opal, despite his denying it and the opal never being found. It turns out she'd misplaced it inside a trick box with a secret compartment and only found out when someone else saw the box and explained the trick. To her credit she is horrified and urgent to set things right as soon as possible.
- Don't Fear The Reaper: Mr. Satterthwaite is unable to save a few people from death in "The Bird with the Broken Wing" and "Harlequin's Lane", but Mr. Quin counsels him that death is not the worst thing that can happen.
- Driven to Suicide: The stories have lovers in dire straits as an integral part of the premise; consequently, a disproportionate number of them include somebody admitting, explicitly or tacitly, that they were on the brink of ending it all when Mr. Quin and Mr. Satterthwaite straightened things out.
- Mr. Quin also intervenes (on request, in his words) in stories where suicides have occurred ("The Coming of Mr. Quin", "The Man from the Sea").
- The final fate of antagonist of "The Face of Helen".
- The central character of "Harlequin's Lane" goes through with it in the end; when it comes to love, not all problems have a neat solution that includes everybody.
- Evil Genius: One character was a brilliant chemist who specialized in poison gas during WWI. He took his love interest ignoring him very badly, sending her a glass vase that includes a glass bubble filled with gas, and arranging for a Glass-Shattering Sound.
- The killer in "The Bird with the Broken Wing" is a mathematical genius who writes papers of such brilliance that 99% of humanity has no hope of understanding them.
- Face Framed in Shadow: In Quin's appearances there's often a "mask" made of shadow falling across his eyes.
- Genre Savvy: The murderer in "At the Crossroads" consciously invokes a number of mystery fiction tropes in an attempt to be regarded as that suspect who looks really guilty but you know didn't actually do it.
- Genteel Interbellum Setting: The stories are primarily set in the 1920s.
- Glass-Shattering Sound: The key to the murderer's plan in "The Face of Helen".
- Go Mad from the Isolation: The criminal of "The Bird with the Broken Wing" is a genius whom nobody noticed or could understand, which caused him to snap.
- Good Is Old-Fashioned: Mr. Satterthwaite is seen as this. Though slightly antiquated, he has a shrewd insight and judgement of human nature, which Mr. Quin relies on.
- Grande Dame: Satterthwaite is friends with a few.
- I Regret Nothing: Played with. At the end of "Harlequin's Lane", Mr. Satterthwaithe is asked if he regrets having had no one to love. He reluctantly says he doesn't. At the time, he was at the site of a lover's suicide, which he could not stop.
- Maybe Magic, Maybe Mundane:
- Mr. Quin, who has a knack for showing up just when something is about to happen, and of saying or doing just the right thing at just the right moment to nudge events in the direction of a happy conclusion. It's heavily implied that he's something other than human, but he never does anything unambiguously supernatural.
- Spiritism is sometimes used to deliver crucial plot information or messages (but not the whole plot).
- Meaningful Name: Mr. Harley Quin, in concert with the Commedia dell'Arte theme.
- Mr. Exposition: Mr. Satterthwaite has a talent for describing the backstory lucidly and with the occasional poetic touch and is frequently called on to exercise it. Played with a bit in that this is also his main detective talent; in the course of describing the situation to another character (and to the audience), he will often notice a detail or correlation of details that points to the solution.
- No Full Name Given: Mr. Satterthwaite's given name is never revealed.
- Offscreen Inertia: The Mysterious Mr. Quin ends with Mr. Quin and Mr. Satterthwaite parting ways after an argument. Christie eventually wrote "The Harlequin Tea Set", specifically to establish that that wasn't the end of their friendship and making a point of ending it with Quin assuring Satterthwaite they will meet again.
- Once More, with Clarity!: Many cases involve Mr. Quin being told the events of the case, and then retelling those same events in a way that reveals the truth.
- Psychopomp: Mr. Satterthwaite considers Mr. Quin an advocate for the dead.
- Sad Clown: Mr. Quin is often described as mocking yet melancholy. Tellingly, he says his favourite opera is Pagliacci.
- Sanity Slippage: The criminals of "The Voice in the Dark" and "The Bird with the Broken Wing" suffer this.
- "Scooby-Doo" Hoax: One murderess managed to get rid of an incriminating bloodstain by going to the room with a jug of hot water. It just so happened that the house was said to be haunted by the ghost of a woman with a silver ewer...
- The Spook: Mr. Quin. Nobody knows where he came from and how is he able to help Satterthwaite's investigations, and he is strongly implied to be supernatural.
- Stealth Hi/Bye: Mr. Quin frequently appears out of nowhere and disappears in a blink of an eye.
- Stopped Clock: A prominent clue in "At the Crossroads".
- Trauma-Induced Amnesia: One murderer is motivated by having regained a set of memories lost in a traumatic event, and discovered thereby that they've been cheated and robbed by someone they trusted.
- You Just Told Me: As Mr. Quin generally can't act directly, he uses facts told to him by others and relays them back with his own deduction of them, which provide the solution.