Trama

A wealthy industrialist's plan to snare a German sympathizer goes awry when the secret plans for a new fighter plane inexplicably disappear.

Tommy Mayfield is a British industrialist who has developed a new fighter plane. Up to now he has paid all development costs himself and he very much wants the government to contribute. It is reluctant to do so because of an apparent indiscretion some years before when Mayfield was supposed to have sold heavy artillery to the Japanese. In order to get back into the government's good books, he decides to lay a trap for Mrs. Vanderlyn, a known Nazi sympathizer and possible spy. The bait is the plans for the new fighter but Mayfield's wife is so concerned that she asks Hercule Poirot to spend the weekend at their house to make sure Vanderlyn doesn't get away with anything.—garykmcd

Hercule Poirot is hired by Lady Mayfield to observe her husband's business dealings. Her husband is Tommy Mayfield, industrialist and aircraft designer. He has just designed and built a revolutionary new fighter, a fighter that surpasses anything the Germans have. However, in order to complete the project he needs funding and has invited the wealthy Mrs Vanderlyn to his estate to discuss her investing in the project. However, Mrs Vanderlyn is a known Nazi sympathizer and there are fears she may steal the plans of the plane for the Germans. Sure enough, soon after her arrival, a crucial part of the plans goes missing...—grantss

The episode starts with a lady approaching Poirot claiming she is Lady Mayfield (Ciaran Madden) & her husband is involved in projects of national defense (he is an arms manufacturer). She is worried that Mr Charles Mayfield (John Stride) has invited Mrs Vanderlyn (Carmen Du Sautoy) to a house party when she is known to be a spy. Lady Mayfield is worried that Lord Mayfield, who is angry with the Govt over the failed business with the Japanese several years earlier (Japan had invaded Manchuria & Mayfield's company was suspected to have supplied arms to the Japanese. The Govt were wrong, but by then the damage to Mayfield's reputation had been done), might be less careful than he should be & that might lead to harm for England.

A house party is underway at the home of Lord Charles Mayfield, a rising politician and a millionaire whose riches came from his engineering prowess. With him are Air Marshal Sir George Carrington (John Carson), his wife Lady Julia (Phyllida Law) and son Reggie (Guy Scantlebury), Mrs. Vanderlyn, who is a beautiful blonde American woman, and Mrs. Macatta, a forthright MP. Mr. Carlile (Albert Welling), Lord Charles' secretary, joins them for dinner. Poirot arrives as a guest of Lady Mayfield.

The reason for the house party becomes obvious when all but Lord Mayfield and Sir George leave the dinner table, as they will discuss the plans for a new bomber aircraft that would give Britain supremacy in the air. Lord Charles has run out of money to develop the bomber further & needs Govt support, but Sir George has told him the Govt doesn't trust him fully yet. They discuss Mrs. Vanderlyn (an American with German sympathies), who is involved in espionage. Lord Charles invited her to tempt her with something big - the bomber - to trap her once and for all & gain the Govt's trust in the process.

Poirot finds that Lady Julia in under gambling debt, & thus has motive to steal the plans. He also finds that inspector Japp is in town & wonders who asked him to be there. All guests retire for bed except Lord Charles and Sir George. Mr. Carlile is sent to get the plans for the bomber from the safe, so he set off for the study, colliding with Mrs. Vanderlyn who wants to retrieve her book. The two men walk along the terrace, when Lord Charles is startled by a figure leaving the study by the French window, although Sir George sees nothing. When they enter the study, Mr. Carlile has the papers out, but Lord Charles quickly sees that the plans of the bomber itself are gone. Carlile is adamant that they were in the safe and he put them on the table. Sir George summons Inspector Japp & wastes no time in arresting Mrs Vanderlyn. No papers are found on her & she is released. Investigating the grass leading off the terrace, Poirot confirms that there are no footprints, which means that someone in the house committed the theft, and the papers are still there.

The next morning the guests begin to leave. Carlile puts a briefcase in Mrs Vanderlyn's car & she gives him a letter to be posted, all under the watch of Lord Mayfield. Poirot is suspicious & follows Mrs. Vanderlyn, who hands over the briefcase to the residence of the German Ambassador. Poirot returns to Lord Mayfield's & confronts him. He claims that Lord Mayfield had indeed helped the Japanese & had written a letter to that effect which was in possession of Mrs Vanderlyn. she blackmailed Mayfield to give her the bomber plans in return for the letter. So, Lord Mayfield staged this elaborate farce to pretend that the papers were stolen, when they were only on his person the whole time. He then handed the papers to Vanderlyn via the briefcase & in return she gave him the letter via Carlile.

Poirot has no doubt that the plans the Germans hold are subtly altered to make the design unworkable. Lord Charles confesses to the deception but insists that his motive, refusing to be derailed from leading England through the coming world crisis, is pure.