The Adventure of the Bruce-Partington Plans

The monotony of thick smog-shrouded London is broken by a sudden visit from Holmes’s brother Mycroft. He has come about some missing, secret submarine plans. Seven of the ten pages — three are still missing — were found with Arthur Cadogan West’s body. He was a young clerk in a government office at Royal Arsenal, Woolwich, whose body was found next to the Underground tracks near the Aldgate tube station, his head crushed. He had little money with him (although there appears to have been no robbery), theatre tickets, and curiously, no Underground ticket. The three missing pages by themselves could enable one of Britain's enemies to build a Bruce-Partington submarine…

(Excerpt from Wikipedia)