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The Submarine Plans is a short story by Agatha Christie which was first published in The Sketch in 1923 in the U.K. The story was published in the U.S. in Blue Book Magazine in 1925. In 1951, the story appeared as part of the anthology The Underdog and Other Stories published in the U.S. The story was later expanded (with changes in some plot elements) as The Incredible Theft. In 1974, the original 1923 text was gathered and published as part of the anthology Poirot's Early Cases.


Poirot is summoned urgently to help recover the plans of a revolutionary new submarine which have been stolen.

Plot summary

(may contain spoilers - click on expand to read)

Hercule Poirot is summoned urgently by special messenger late at night to 'Sharples', the home of Lord Alloway, head of the Ministry of Defence and a potential future Prime Minister. Travelling there with Hastings, he is introduced by their host to Admiral Sir Harry Weardale, the First Sea Lord who is a guest at Sharples together with his wife and son, Leonard. The reason for the summons is that the secret plans for the new 'Z'-type of submarine were stolen some three hours earlier.

The facts of the case are that the ladies of the party – a Mrs Conroy and Lady Weardale – retired to bed at 10:00 pm as did Leonard Weardale. Lord Alloway instructed his secretary, Mr Fitzroy, to place the various papers that he and the Admiral would require to do their evening's work out on the table in the study while the two men walked on the terrace and finished their cigars. Lord Alloway fancies that as they turned back on the terrace he saw a shadow move away from the open French window that leads into the study, although the Admiral is dismissive of this idea. Going into the study, the submarine plans, having been moved by Fitzroy from the safe to the table, were gone. Fitzroy himself had been distracted by a scream from the stairs outside the study and had gone out to find Mrs Conroy's maid claiming that she had seen a ghost. It was then that the plans were apparently stolen.

By torchlight, Poirot examines the grass outside the study window and finds no trace of footprints, even though it rained earlier in the evening and that means someone in the house is responsible. He questions all of the people concerned although at Lord Alloway's insistence, he doesn't reveal to them that the plans have been stolen. Fitzroy is discounted as, Lord Alloway points out, the man has access to the safe and could have copied the plans at any time. The prime suspect is Mrs Conroy whose past life is something of a mystery and who moves in diplomatic circles. She was specifically asked down for the weekend in order that they could keep an eye on her. Questioning her maid, Poirot correctly guesses that she screamed when Leonard Weardale sneaked an unexpected kiss from her and she quickly made the story up when her cry attracted Fitzroy's attention.

At this juncture, Lady Weardale reappears and asks Poirot if the matter could be dropped if the plans were returned. He agrees that that could be arranged and she promises that they will be within ten minutes. Poirot puts this offer to Lord Alloway, without mentioning any names, and leaves.

Driving back, Hastings questions this unexpected turn of events, suspecting Lady Weardale's rumoured bridge debts are real. Poirot points out that as the story of the shadow of an intruder leaving the study turned out to be incorrect, the plans must have been taken by Fitzroy (who they have previously discounted) or by Lord Alloway himself – the logical conclusion. Hastings is dubious of this deduction but Poirot points out that the man was rumoured to be involved in share scandals years before, although he was later exonerated, but suppose the rumours were true and he was being blackmailed, in all probability by Mrs Conroy, a foreign agent? He would hand over fake copies of the plans to her, with suitable adjustments in them to make them useless, and then pretend they had been stolen while all the time ensuring that his loyal secretary wasn't suspected. Leonard Weardale was busy with the maid at the time of the supposed theft and Lady Weardale couldn't be the thief as she needed ten minutes to get the plans back whereas she would need a lot less time than that if she had taken them and hidden them. The theory Poirot has also explains why Lord Alloway didn't want the guests told of the theft of the plans – he wanted the foreign power to receive the false plans.

Hastings remains unconvinced but on the day in the future when Lord Alloway becomes Prime Minister, Poirot receives a cheque and a signed photograph from him, dedicated to "my discreet friend". He also hears that a foreign power attempted to build their own version of the submarine and it ended up a failure.


References to other works

Film, TV, or theatrical versions

This short story was later expanded into the novella-length story The Incredible Theft with changes to the character names and some story details (e.g., the plans are for a new state-of-the-art bomber rather than for a new submarine). The Incredible Theft was adapted as a television film with David Suchet as Poirot in the ITV series Agatha Christie's Poirot, first broadcast on 2 February 1989. Because of this, The Submarine Plans, which has a very similar storyline was never adapted for the series.

Publication history