The Submarine Plans

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

The prime suspect is a Mrs Conroy, whose past life is something of a mystery, and who moves in diplomatic circles. Questioning Conroy's maid, Poirot correctly guesses that she screamed when Leonard Weardale sneaked an unexpected kiss from her. Lady Weardale 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 points out that the plans must have been taken by Lord Alloway himself. He theorises that Alloway was being blackmailed, in all probability by Mrs Conroy, a foreign agent. He handed over fake copies of the plans with suitable adjustments to make them useless, and then pretended they had been stolen.

Later, when Lord Alloway becomes Prime Minister, Poirot receives a cheque and a signed photograph dedicated to "my discreet friend". He also hears that a foreign power attempted to build their own version of the submarine which ended up a failure.