The Adventure of the Bruce-Partington Plans by Arthur Conan Doyle

When top-secret submarine plans go missing and a government clerk is found dead near the railroad tracks, Holmes is called in to avert a diplomatic disaster. Following a trail of espionage, betrayal, and codes, Holmes exposes a traitor with help from his brother Mycroft. The plot blends politics, technology, and subtle deduction. It’s one of Holmes’s most significant national-security cases. An intricate mystery with high stakes and international tension.