Watson says that the previous story was supposed to be the last one, but he had to add this crazy one as the grand finale.
This case involves important politicians and a lot of the details have to remain top secret.
We also learn that in the present day, around 1903, Holmes is retired and is working as a beekeeper in Sussex. Awesome.
At some unnamed date, Lord Bellinger, the Prime Minister, and Trelawney Hope, the European Secretary, arrive at Baker Street.
They need Holmes to work a super important case and to prevent a war from breaking out.
Trelawney Hope has lost a letter from a foreign ruler and if that letter falls into the wrong hands it could be used as an excuse to start a war.
We never learn who the foreign ruler was, but the fact that he is referred to as a "potentate" implies that it's the ruler of the Ottoman Empire, present day Turkey mainly, or Russia. Potentate has connotations, or meanings, that can be linked to how Europeans viewed "Eastern" or Asian rulers in this time period as despots, or dictators. Also, in the late nineteenth century, Turkey and Russia were both very tense political hotspots and issues there spilled over into western Europe.
Another history snack: The men hear mention that Europe is an "armed camp" at the moment. This is a reference to the types of treaty systems that were common in the late nineteenth and early twentieth centuries. Basically, all the major powers were allied on one of two sides. If any one member of an alliance went to war, then all their allies had to follow, and pretty soon everyone was fighting. This sort of alliance system is basically what caused World War I to break out in 1914 and escalate into a "world war," about a decade after The Return of Sherlock Holmes was published. So there's some interesting political commentary there.
After explaining the seriousness of the situation, Trelawney Hope admits that he can't figure out how the letter was lost or stolen since it was locked up, and no one knew it was there.
Holmes gets some more details on the political situation, the motives involved, and the identity of the foreign ruler. Watson doesn't give us the guy's name though.
Holmes finally says that he doesn't see how he can help since there are really no clues.
The statesmen ask Holmes to do his best and leave.
Holmes says he'll start by tracking down some spies he knows about to see if anyone is selling documents on the black market.
He mentions checking out an Eduardo Lucas.
But Watson says that Lucas is dead, according to the newspaper.
Holmes is stunned, which Watson finds amusing.
The article explains that Lucas was a popular man and was found dead in his house last night. No one knows who killed him or why.
Holmes feels he's on to something big now.
Before he can go investigate, Lady Hilda Trelawney Hope arrives.
She's Hope's wife and she asks Holmes very awkwardly for some details about the documents that are missing.
Holmes refuses to tell her since he won't betray a client.
Lady Hilda begs some more and seems very upset.
Then she leaves.
Holmes and Watson find this super suspicious, as well as weird.
So Holmes rushes off to investigate the Lucas murder.
Lucas's valet, John Mitton, is arrested.
There isn't enough evidence against him, however, and his alibi holds up.
So, he is released.
Lestrade is working the Lucas case.
Finally they get a break. Turns out Lucas had an alternate life in Pars as an Henri Fournaye, and he had a wife there too. The wife found out about this alternate life, was less than thrilled, and followed her husband to London. She confronted him and killed him in a passionate rage. She's now suffered a mental breakdown.
Holmes is frustrated because there are no leads on the missing documents.
But then Lestrade calls Holmes to the Lucas house.
Lestrade noticed something odd – there's a blood stain on the rug that doesn't match up to another blood stain on the floor. The rug was moved at some point.
Lestrade goes to talk with the constable who was supposed to be guarding the murder scene.
While he's gone Holmes frantically looks under the rug and finds a hiding spot. But there are no documents there. He puts everything back in order just before Lestrade returns with the constable, named MacPherson.
MacPherson is shame-faced after having been chewed out by his boss.
He explains that a pretty lady wanted to see the murder scene so he let her in. Then she fainted at the sight of the blood stain.
MacPherson ran off to get her some water or something.
When he came back, she said she felt better and then left.
On their way out, Holmes shows MacPherson a picture and asks if that was the lady. He is surprised and says yes.
Holmes and Watson then head off to throw down with Lady Hilda.
Holmes accuses her outright of having the letter, and she keeps denying it, stupidly.
Finally, she confesses to it and then proceeds to give a really long-winded explanation before her husband comes back. Holmes keeps telling her to hurry up and give him the darn letter already.
In a nutshell, Lady Hilda was being blackmailed and agreed to deliver some government papers to Eduardo Lucas in exchange for some love letters. So she almost started a war because she was embarrassed over love letters. Sheesh.
Anyway, after Eduardo died, she went back, conned MacPherson, and stole back the letters.
It's interesting that this entire potential political crisis hinged upon the actions of two irrational women. We're sensing some gender themes there.
After Lady Hilda finally stops talking, Holmes gets the letter and shoves it back in the lock box before the Prime Minister and Hope return.
Holmes tells Hope to check the box again and the letter is there. It's magic!
Hope is too happy to care that this is weird, but the prime minister thinks Holmes is responsible.
Holmes refuses to say though, and he and Watson head back home to Baker Street.