Poirot speaks with Superintendent Battle, Colonel Race and Mrs. Olivers while the other four guests wait in another room. Battle questions each one. Dr. Roberts Mrs. Lorrimer and Anne Meredith and Major Dethspar all deny any involvement in the murder. Poirot collects the score sheets from the bridge game to mark the passage of time as well as to give clues to the character of each suspect.
As the investigation proceeds, each sleuth discovers a murder. Battle finds out that a client of Dr. Roberts, along with the client's spouse, died separately, one of anthrax, the other of blood poisoning. Colonel Race reveals that Despard led botanist Luxmore through the Amazon jungle where the latter died of fever, with rumours he was shot. Mrs. Oliver learns that a woman who employed Anne as a companion died of accidental poisoning. Poirot uncovers the fact that Mrs. Lorrimer poisoned her husband. Colonel Race leaves the country for his work in the Secret Service. The reactions of the guests vary: Anne is afraid, Despard engages a lawyer and Dr. Roberts carries on as usual.
Mrs. Lorrimer, who admits she killed her husband, says she has a fatal health condition and that she is the one who killed the host. However, she is not believed; it appears she is trying to spare Anne. Anne comes to visit Mrs. Lorrimer and the following morning the latter is found dead of a sleeping drug overdose. However, when Poirot comes upon the scene he sees a hypodermic needle mark on Mrs. Lorrimer's arm.
Anne takes her flatmate Rhoda out in a boat on the nearby rivers as they await a visit from Despard. Poirot and Battle race to Anne's cottage where they see Anne deliberately tip Rhoda out of the boat, but the latter pulls the former into the water as well, and neither can swim. Despard saves Rhoda and then Anne. Rhoda survives but Anne dies.
At Poirot's apartment he presents his theory on the murders. The sleuth presents a window washer who saw Dr. Roberts inject Mrs. Lorrimer. The police ruled she died of an anesthesia overdose. Dr. Roberts killed Mr. Shaitana as well. He waited until he was a "dummy" in the bridge game and excused himself to get a glass of water. Furthermore, Doctor Roberts had killed Mr. Craddock, the husband of one of his patients, by putting anthrax on his shaving brush during a house call. Then he injected Mrs. Craddock with her required anti typhoid injection before her trip to Egypt but added a germ which led to her fatal blood infection. Roberts at first protests but eventually admits he is guilty. The window washer was actually an actor used to solicit the confession from the doctor.
Major Despard is exonerated when it is proven that the botanist died from an accident shooting wound. Despard ends up courting Rhoda, Anne's flatmate.
Cards on the Table published in 1936 courtesy