[1] On a rainy night, a frantically crying man (Percy Phelps) is carried into his home by a few men while his fiancée (Annie) and her brother (Joseph Harrison) watch over him worriedly.
Phelps writes that after completing his education he obtained an appointment at the Foreign Office and that a misfortune has occurred which could ruin his career and reputation.
Holmes deduces that the letter was written by a woman "of an exceptional nature" and agrees to accompany Watson to Phelps’ home, Briarbrae House in Woking.
Phelps narrates the incidents of the evening of 23 May, when he was summoned by Lord Holdhurst (the Foreign Minister and his uncle) to his office.
Lord Holdhurst entrusted Phelps with the task of creating a copy of a naval treaty signed between England and Italy.
In answer to Holmes' question, Percy tells him that the document defines the position of Great Britain towards the triple alliance in the event of the French fleet gaining ascendency over Italy in the Mediterranean.
Phelps tells Holmes that he had intended to join Harrison on the 11 o'clock train back to Woking but was not able to because the document was very long.
Holmes and Watson travel to the Foreign Office, where they meet Inspector Forbes, who tells them they have cleared both the commissionaire and Phelps' colleague Gorot.
At Briarbrae House, Phelps sends the hired nurse away and tells Annie that he will sleep without medication, since Holmes' taking the case has given him new hope.
On the way to Woking station, Holmes suddenly gets out of the carriage, telling Watson to take Phelps to Baker Street and remain there and that he expects to join them by breakfast time the next morning.
Holmes spends the day in the countryside, returns to Briarbrae later in the evening and sees Annie reading in Phelps' room.
Phelps lifts the lid off a dish and finds, to his immense surprise and joy, a scroll of paper which turns out to be the stolen treaty.
Holmes reasoned that since the attempt to break in was made on the night that the hired nurse was not present, the person must have known the household well.