The Adventure of the Second Stain by Arthur Conan Doyle

When a top-secret diplomatic letter goes missing from a government office, Holmes is called to prevent international conflict. As he investigates, the case leads to blackmail, hidden affairs, and a murder that turns out to be secondary to the main scandal. The plot involves multiple layers of deception and political intrigue. Holmes ultimately protects national security with discretion and logic. It’s one of the more high-stakes and politically charged Holmes stories.