Renowned actor Sir Charles Cartwright has rather improbably retired to the small, nondescript Cornish fishing village of Loomouth. As the novel opens, he is hosting a house-party of thirteen people, a mixture of local and London friends including Poirot and Satterthwaite. No sooner are the initial cocktails served than one of the guests, the local Vicar Stephen Babbington, suddenly falls down dead. Despite Sir Charles' suspicions of foul play—which his friends dismiss as a quite natural hankering after melodrama—and given that Babbington was an elderly man in poor health and with no enemies, the death is officially recorded as "natural causes".
However, Sir Charles is proven right not long afterwards, when Dr Bartholomew Strange, an old friend of Cartwright's and a guest at the fatal house-party, dies in the exact same manner as the vicar while hosting his own house-party... which just so happened to feature a nearly identical guest list. And this time, it's unquestionably nicotine poisoning. A subsequent re-examination of Babbington's body finds more of the same poison.
With the help of love interest Hermione "Egg" Lytton-Gore, Mr Satterthwaite and, eventually, Hercule Poirot, Sir Charles embarks on a grand quest to figure out the connection between the two incidents, plus a third that also appears to be highly meaningful to both. In the process they uncover one of the strangest motives for murder in Poirot's experience.
Notable in the Christie canon as one of only two novels that exist in substantially different UK/American editions (the other being the Miss Marple story The Moving Finger). A friend of Christie's was reportedly vocally dissatisfied with the weakness of the original solution to Three Act Tragedy, whereupon Christie reread, agreed, and reworked it more or less completely for a new edition, while making only very minor changes to the clues as planted. Nowadays, UK editions retain the original solution, and their American counterparts the revised one.
The novel was adapted into a 1986 film called Murder in Three Acts, starring Peter Ustinov, and then into a 2010 episode for the twelfth season of ITV series Poirot. Tropes for the 2010 adaptation can be found on the ITV series page, while the 1986 film has its own section below.
The original novel provides examples of the following:
- Affectionate Nickname: Charles calls his Childhood Friend Dr. Bartholomew Strange "Tollie".
- Amateur Sleuth: The investigation into Babbington and Strange's deaths is initiated by Charles Cartwright, Egg and Mr. Satterthwaite, and they rapidly discover more clues than the local police, much to the latter's chagrin. Of course, one of them is the culprit after all, and the clues they found were deliberately meant to mislead.
- The Butler Did It: Zigzagged. After the second death, the victim's new butler, Ellis, immediately disappears, implicating himself as the culprit. After some poking around Ellis' room, however, Satterthwaite and Cartwright find various drafts of a letter in which the butler seems to be figuring out how to blackmail the real murderer. In the final act, Poirot reveals the truth: the butler is the culprit after all. Only he's not a real butler, he's Sir Charles Cartwright disguising himself as Dr. Strange's butler, with the doctor convinced it's all an elaborate practical joke on their mutual friends.
- Chekhov's Party: Invoked and played with. Everyone assumes that Sir Charles's first party is this, where Babbington died, and that it holds the solution to the second party that ends in murder. But it doesn't - the first party is merely a trial run that serves to throw people off the scent. The second one is the important one.
- Demoted to Extra: Even though the novel is considered to be a part of the Hercule Poirot series, the Belgian detective's involvement in the case mostly consists of his insistence that there isn't a case, until the third act.
- Embarrassing Last Name: Cartwright is a stage name. Sir Charles's real last name is "Mugg", a fact he's understandably not proud to admit.
- Eyes Never Lie: Mr Satterthwaite wonders how Muriel Wills could write the plays credited to Anthony Astor, until he makes eye contact with her and is alarmed by the keenness and intelligence of her gaze. It feels to him "as though Miss Wills were painstakingly learning him by heart."
- He Knows Too Much:
- Led by Sir Charles, our amateur detectives assume that Dr. Strange was killed because he knew the truth about the first murder, was obviously about to test his theory out on the same batch of houseguests, and thus had to be silenced in a hurry. This turns out to be a Red Herring, as the doctor knows nothing about the first murder... but he is killed because he knows a secret that the killer wishes to hide.
- Prior to her death, the third victim sent a telegram to the sleuths, telling them that she knows something about the murder case. However, she was killed before anyone could find out what she knew. This was a rare case of inversion of the trope. The note was a false clue sent by the killer to confuse the detectives, and the victim was actually killed to prevent the others from finding out that she actually knows nothing.
- In-Series Nickname: Hermione Lytton-Gore is known to every other character as "Egg". According to her mother, the nickname came from her toddler years, when she was rather roly-poly.
- Inferiority Superiority Complex: Oliver Manders often acts conceited and frequently rubs others' faces in his uncle's wealth. The more perceptive characters (Mrs. Lytton-Gore, Mr. Satterthwaite and Poirot) are however convinced that it's all a facade to hide his insecurities over being an illegitimate child.
- Jumping Off the Slippery Slope: The revised (now American) ending changes the killer's motive into this. Sir Charles is suffering from a growing megalomania, but as he's already known as a flamboyantly theatrical man he can hide it from everyone except Dr. Strange, who besides being his closest friend is a 'nerve specialist' (psychiatrist). Sir Charles thus murders the doctor out of fears that he might commit him to an insane asylum.
- Karma Houdini: An ambiguous example; Sir Charles flees at the end, and Poirot makes no move to stop him, but remarks that there's really no escape for him and all he can really do is "choose his exit", with the implication being that he's either going to kill himself off-page or will be apprehended, and even if he doesn't he's been exposed and ruined.
- Love Makes You Evil: In the original ending. Sir Charles wants to marry Egg, but is unable to do so because he can't divorce his first wife, since she's certifiably insane. In order to get around this, he murders his childhood friend Tollie Strange because he's the only person who knew of his first marriage.
- MayDecember Romance: Charles Cartwright, 55, is in love with Egg, 25, though he fears that she prefers her old friend Oliver Manders, who is much closer to her age. Egg, on her part, hero worships Sir Charles and happily accepts his attentions. Not knowing that he has murdered three people to secure their marriage certainly helps.
- Moustache de Plume: In-universe. Anthony Astor, a currently fashionable playwright, is actually a very conventional middle-aged woman by the name of Muriel Wills. Many people are surprised to find out that a woman who "looks exactly like a rather inefficient nursery governess" is behind "his" fabulously witty, sharply-observed plays.
- Mystery Magnet: Discussed by the characters in the first chapter, in reference to the way Poirot has of stumbling into murders. Sir Bartholomew has a theory that events come to people, not people to events — a man may travel all over the world and just barely miss anything bizarre going on, while another man may live in a London suburb and find himself caught up in all sorts of intrigues. "In the same way, men like your Hercule Poirot don't have to look for crime; it comes to them."
- Serial Killings, Specific Target: Played with. Three highly unlikely people are killed by nicotine poisoning, the first two in exactly the same manner and surrounded by the same people—given that it's not coincidence, what's the connection? The first murder turns out to have been merely a sort of dress rehearsal for the method used in the second, with the victim chosen absolutely at random. The second murder is the significant one, while the third serves to cover that fact up.
- Supporting Protagonist: The main viewpoint character of the novel is Mr. Satterthwaite, who very characteristically brings Poirot into the investigation while he himself mostly serves as a background observer.
- Trial Run Crime: Babbington dies entirely at random so the killer can practice his method of poisoning and cleaning up the evidence in the confusion.
- Who Murdered the Asshole?: Inverted. Both the first two victims are very nice people whom practically everyone likes, the third isn't known to any of the main cast at all, and so trying to figure out the murder motive is the main stumbling block to the investigation. There's even more to it: as it turns out in the end, the killer didn't hold any grudge whatsoever against the victims either.
The 1986 film, Murder in Three Acts, additionally contains examples of the following:
- Adaptational Name Change: Hermione and Mary Lytton-Gore become Jennifer and Daisy Eastman, Oliver Manders becomes Ricardo Montoya, Emily Wills becomes Janet Crisp, Bartholomew Strange is renamed Wallace, and Angela Sutcliffe's surname is changed to Stafford.
- Adaptational Nationality: Sir Charles Cartwright, an English actor in the original novel, is made into an American movie star played by Tony Curtis.
- Adapted Out: Mr. Satterthwaite does not appear, and his role is taken by Hastings.
- Setting Update: Similarly to Thirteen At Dinner, the time period of this film is updated to the 80s. Also, the location is changed to Acapulco, Mexico; while its heyday as Hollywood's getaway of choice was in the 1950s, in the 1980s it was still seen as a glamorous destination - the sort of place an American movie star might retire to.