"Bah!" Mr. Shaitana snapped disdainful fingers. "The cup used by the Brighton murderer, the jemmy of a celebrated burglar — absurd childishness! I should never burden myself with rubbish like that. I collect only the best objects of their kind."
"And what do you consider the best objects, artistically speaking, in crime?" inquired Poirot.
Mr. Shaitana leaned forward and laid two fingers on Poirot's shoulder. He hissed his words dramatically.
"The human beings who commit them, M. Poirot."
A neat trick, or a grand slam? Agatha Christie's novel Cards on the Table presents a mystery of an unusual shape. The solution to the murder lies not so much in establishing alibis and time of death, tracking footprints or the like, but within a process of superb psychological detective work.
Her Belgian detective, Hercule Poirot, is invited to a London dinner party, among a most oddly chosen group of guests. The host has invited eight to his party, and with devilish glee informs Poirot that of the eight diners, four are more or less specialists in detection — Scotland yard; a mystery novelist; and so on. The other four are murderers, or so says the host.
After dinner, the host insists on bridge being played, and divides the guests into two tables of four,, in two rooms; himself sitting out. Before the evening is over, a murder has been committed. All of the suspects are reasonably suspectible.
At an exhibition of snuff boxes, Hercule Poirot meets Mr. Shaitana, a mysterious foreign man who is consistently described as devil-like in appearance and manner. Shaitana jokes about Poirot's visit to the snuff box exhibition, and claims that he has a better "collection" that Poirot would enjoy: individuals who have gotten away with murder. He arranges a dinner party to show off this collection; Poirot is apprehensive.
Upon arrival at Shaitana's house on the appointed day, Poirot is joined by three other guests: mystery novelist Ariadne Oliver, Scotland Yard's Superintendent Battle, and Colonel Race of His Majesty's Secret Service. Soon, the other four guests join them: Dr. Roberts, a hearty, florid man; Mrs. Lorrimer, a perfectly poised gentlewoman of late middle age; Major John Despard, a dashing Army man and world traveler, recently returned from Africa; and Anne Meredith, a shy, quiet, very pretty young woman. Having brought them all to dinner, Shaitana skillfully manipulates the topic of conversation to possible motives for murder. He lists four particular examples: the idea of poison as a woman's weapon, a crime of passion, the "accidental" murder, and a doctor's seemingly limitless opportunities to dispose of patients. These comments cause an unsettling air to fall over the dinner party; Mrs. Oliver comments that a "black angel" is passing.
Shaitana invites his eight guests to play bridge in the adjoining rooms; he, as the odd man out, does not play. Roberts, Meredith, Lorrimer, and Despard play in the first room, while Poirot, Oliver, Race, and Battle play in the next; Shaitana settles himself in a chair in the first room and thinks of how wonderfully his party is going. Hours later, Poirot and the others prepare to leave, and go to thank Shaitana. The other four are still in the middle of their game, but Battle notices that Shaitana has stopped moving, and that his body is slack. Shaitana has been murdered, stabbed in the chest with a jeweled stiletto. Battle interrupts the card game to announce this; the four players quickly say that no one else has entered the room all evening, meaning that one of them must be the murderer.
Once the preliminary police work has been done, Poirot reveals Shaitana's strange mention of a "collection" to the other three with whom he played bridge. They quickly realize that they are four "sleuths" meant to be pitted against the four in the next room whom Shaitana suspected of murder. The four agree to work together to solve the crime, and interview the four suspects. Roberts is upset but still hearty; Mrs. Lorrimer remains unshaken; Despard is smooth and does not pretend to be upset; Anne Meredith is the most ill at ease. Poirot takes interest in the way each member plays bridge, which he discerns through asking each suspect to grade the play of the others, ultimately determining that Lorrimer is the most skilled player, while Despard is talented but plain; Meredith plays too carefully, while Roberts bluffs and overbids often. As there seems to be no conventional way to prove which of them has committed Shaitana's murder, Poirot suggests that the group of sleuths delve into the past and uncover the murders that the dead man thought he knew about. He also asks each of the four "murderers" what they remember about Shaitana's ornately furnished front room, and about each rubber of the bridge game, seeking clues in their answers to the personality and character of each suspect.
Battle is put on the trail of the death of a Mrs. Craddock, whom Dr. Roberts once attended. Her husband died of anthrax poisoning from an infected shaving brush (and readers at the time of the novel's publication in the 1930s might well have remembered anthrax deaths from infected shaving brushes during and in the years after World War I); Mrs. Craddock herself had died not long afterward, of a tropical infection, in Egypt. Race seeks out information on Despard, and discovers a case in which a botanist named Luxmore and his wife traveled with him to South America; Luxmore officially died of a fever, but it is rumored that he was shot. Mrs. Oliver visits Anne Meredith and her housemate, Rhoda Dawes. Anne is one of those stock characters of the Christie mysteries, the well-bred but penniless orphan; she lives with her old friend Rhoda in Rhoda's house and is dependent on Rhoda's generosity. Mrs. Oliver tells the two that it falls to the women to solve the crime; she lists possible motives, including blackmail, to which Anne reacts badly. Rhoda later visits Oliver and explains Anne's bad manners: Anne, after her father's death and before old friend Rhoda came to her rescue, worked as a live-in companion (in an era in which a "companion" was a paid personal attendant of a wealthy woman); one employer, a Mrs. Benson, had taken hat paint—poison—from a medicine bottle and died. It was Anne who had broken the bottle of hat paint and placed the remainder in an emptied medicine bottle, at Mrs. Benson's suggestion, the story went. Though all in the remote village agreed that Anne was blameless and the death accidental, any mention of a murder abashes her. Fellow suspect Despard takes an interest in Anne's welfare, recommending that she retain an attorney.
In the meantime, the four sleuths gather and compare notes. Battle discovers that Roberts may have had an illicit relationship with Mrs. Craddock, while Poirot visits with Mrs. Luxmore. The latter is a melodramatic woman who claims to have had an unconsummated but passionate affair of the heart with Despard; yes, he did shoot her jealous husband, but it was in self-defense, she maintains, while histrionically making it clear that she lies to protect her love. Despard refutes all that. Mrs. Luxmore, he says, had persuaded herself that she was in love with him, but he took no interest in her, and killed Luxmore by accident while attempting to stop the man, who was delirious with fever, from wandering into the river and drowning. If Mrs. Luxmore hadn't thrown herself at Despard's gun, her husband would only have been wounded in the leg. Meanwhile, Poirot sets a trap for Anne Meredith. He knows that she is a poor girl with a hunger for expensive things. When she pays him a call at his request, he shows her to a table on which many packets of the finest silk stockings are piled up, apparently carelessly, and asks her to do him a favor by selecting those that would be best for him to give at Christmas. (Such stockings would truly have been made of silk; nylons hadn't yet been invented.) Poirot deliberately understates the number of packets in the pile, then wanders off to another room, leaving Anne alone with the stockings. After Anne makes her gift suggestions and leaves, Poirot discovers that two pairs of the stockings are missing, confirming his suspicion that Anne is a thief, and seemingly giving weight to his suspicion that she stole from Mrs. Benson and killed her when she feared she had been discovered.
At this point, Mrs. Lorrimer contacts Poirot with surprising news. She confesses to Shaitana's murder, and explains that she took the stiletto impulsively after he mentioned poison as a woman's weapon. Shaitana was right about her, she says; twenty years earlier, she had, she confesses, killed her husband. Poirot objects that Lorrimer's explanation of Shaitana's killing does not match her unflappable personality; she might well commit a premeditated murder, but never an unplanned, spur-of-the-moment one. Lorrimer breaks down—a trifle too easily for one presented to the reader as a model of self-possession—and admits that she saw Anne Meredith standing over Shaitana with her hand on his chest. Lorrimer thus believes that Meredith is Shaitana's killer, and decided to lie to save the younger woman. Lorrimer can do this because she has learned that she is terminally ill. She begs Poirot to let her take the blame for the crime: she will die soon anyway, and Anne will be free to live her young life.
Poirot is confused by this confession, and fears that there may be more trouble to come. His guess proves correct when Mrs. Lorrimer is found dead the next morning, having apparently committed suicide. Roberts arrived before she was quite dead and attended to her, but she could not be saved. Poirot and Battle race to Anne Meredith's cottage, fearing that she might strike again. Despard, who has been visiting Anne and Rhoda, both of whom fancy him, is a few steps ahead of Poirot and Battle. At Anne's suggestion, Anne and Rhoda are on a boat in a nearby river. Anne knows that Rhoda visited Mrs. Oliver, but does not know that Rhoda revealed the story of the Benson death to her; Anne believes that Rhoda is the only person likely to cross paths with the London police who can connect her to that death. Poirot and Battle see Anne suddenly push her friend into the water. Alas for Anne, when she knocks Rhoda into the water, she also falls in herself. Ah, but Despard has arrived; he must choose between the foundering young women; his choice will reveal which he loves. He rescues Rhoda; Anne drowns.
Poirot gathers Oliver, Battle, Despard, Rhoda, and Roberts at his home, where he makes a surprising announcement: the true murderer of both Shaitana and Mrs. Lorrimer is not Anne, but Dr. Roberts. Poirot brings in a window cleaner who happened to be working outside Mrs. Lorrimer's flat earlier that morning. He testifies that he saw Roberts inject Lorrimer with a syringe; a syringe, Poirot reveals, full of a lethal anesthetic. Battle chimes in that they can bolster any prosecution with the true story of the deaths of the Craddocks, who died of infections, true, but infections deliberately inflicted on each of them by Roberts. Roberts confesses.
Poirot proudly explains his reasoning to the gathered group. He points out that in the third rubber of bridge on the night of Shaitana's murder, a grand slam occurred. This intense play would keep the others focused on the game—Roberts was dummy at that point—while Roberts used the opportunity to stab Shaitana. It is also revealed that the "window cleaner" was actually an actor in Poirot's employ, though Poirot brags that he did "witness" Roberts kill Mrs. Lorrimer in his mind's eye. Despard ends the novel on a cheerful note: he suggests that one of the gathered party murder Poirot, and then watch his ghost come back to solve the crime.