Arthur Cadogan West, a young government clerk, is found dead by the railway tracks near Aldgate. On his body Holmes' brother Mycroft finds some secret submarine plans - three pages of which seem to be missing. These pages alone, in the wrong hands, could be used to build a Bruce-Partington submarine that would pose a threat to Britain. Cadogan West's head has been crushed, but it looks like there has been no robbery - was the assailant after the plans? On further investigation, with no evident signs of a struggle or violence inside any of the railway coaches or track-side, our heroes realise that this is definitely no straight-forward mugging. It is clear that the young man planned to sell the plans for profit, but how was he killed, and why, exactly? Lucky that Holmes, Watson, Mycroft and friends are on the case - rest assured they'll find their reason - and their man.