Black Coffee (1973)

The inventor Lord Amery has developed a new chemical formula that could make a great deal of money. But during a dinner with his family, the formula is stolen from the safe in his study. After dinner, as coffee is served in the library, Lucia complains of a headache. Barbara then takes a medicine cabinet down from a shelf. Dr. Carelli tells the others that it contains very strong painkillers that can be fatal in an overdose. Lucia secretly takes some tablets and dissolves them in one of the coffee cups, making sure that Lord Amery is brought a different cup. The Lord has a plan to find the thief who stole the formula: he has the doors to the library locked from the outside. At nine o’clock sharp, all the lights will be switched off briefly so that the thief can put the formula back on the table without being recognized. If he does not, Detective Hercule Poirot will be put in charge of the case; he is already on his way to the Lord’s castle. When the lights come back on, an empty envelope, with no formula inside, is lying on the table, and the Lord is sitting dead in his armchair. The Lord’s family doctor, Dr. Graham, determines that he has been poisoned, so Poirot and his assistant Hastings set out to investigate who might have murdered him. The following people were in the library and are suspects: Caroline, Lord Amery’s sister; Richard, his son; Richard’s wife, Lucia, an Italian woman; the Italian doctor Dr. Carelli, whom Lucia knows from Italy and who arrived unexpectedly that afternoon; Raynor, the Lord’s private secretary; and Barbara, Lord Amery’s niece. As Poirot’s interviews the following day reveal, Lucia had intended to poison herself because Dr. Carelli was blackmailing her: Carelli knew that Lucia was not, as she claimed, an orphan, but the daughter of a notorious Italian “woman of pleasure” with a bad reputation. He threatened to reveal the secret of her parentage unless Lucia obtained her father-in-law’s formula for him. He got her a duplicate of the safe key, but she did not dare use it and, when the lights went out, threw the key away. Lucia’s husband Richard was initially jealous of Carelli. But then he promised Lucia that he would get money and emigrate with her, giving her renewed hope, and she did not drink the coffee that had already been poisoned. To reconstruct what happened, Poirot asks Caroline to recount exactly what sounds she heard while the lights were out. He also notices that the medicine cabinet is clean, while the shelf it stood on is very dusty. It must therefore have been taken down earlier, and a second person had taken some poison. During his questioning of the secretary, Raynor, Poirot has him pour a whisky, but secretly swaps it for a glass Hastings has ready. Poirot pretends to have been poisoned and gets Raynor to give himself away: before Lucia, he had already taken tablets from the medicine cabinet, dissolved some in Lord Amery’s coffee, and put some in Poirot’s whisky just now.