The Bruce Partington Plans (S04E05)

When Cadogan West's lifeless body is discovered in an underground tunnel close to Aldgate Station, Holmes is intrigued by his brother Mycroft's unusual involvement in the case. Mycroft reveals that West was in possession of highly classified plans for a new submarine, but tragically, three out of the ten pages are missing. Concerned for national security, Mycroft enlists Holmes to help recover the missing documents and unravel the mystery surrounding West's death.