The Submarine Plans — Agatha Christie

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.