Kathleen Adair, Dr. Watson's lifelong friend, asks his advice about her brother Ronald, a young diplomat with the Foreign Office, who she believes has been cheating at cards. She wants Watson to bring the case to the attention of renown private detective Sherlock Holmes, his good friend. When Ronald is asked to deliver diplomatic documents to France, Moriarity, Holmes's arch enemy, succeeds in capturing him and threatens to expose his cheating if he does not agree to take a package of stolen bank notes to France by diplomatic pouch. Holmes discovers where Ronald is being held and leads a raid on Moriarity's headquarters. He rescues Ronald but Moriarity escapes using one of his many disguises. Rather than transport the bank notes, Ronald decides to report the incident to the authorities, but he is shot before he can carry out his plans. The murder is made to look like suicide, but Holmes deduces that Moriarity was behind an ingenious plot. During break-ins that were made to look like robberies, counterfeit bank notes were planted. Moriarity then committed two murders to cover up this scheme. Narrowly escaping death, Holmes traps Moriarity and reveals his real identity.