当奥地利王储的尸体被发现，雷斯垂德探长（艾迪马森饰）由证据推断是死于自杀，不过福尔摩斯则推论是死于他杀，这起谜团般的不祥谋杀案经过严密设计，而幕后黑手正是莫里亚蒂教授。
福尔摩斯追查线索追到一间地下的绅士俱乐部，他和兄长迈克罗夫特·福尔摩斯（史蒂芬弗雷饰）向华生医生举杯祝酒，替他举办告别单身之夜。福尔摩斯在那 里遇见年轻的吉普赛算命师希姆（欧蜜瑞佩斯 饰），她目睹了一些秘密却隐瞒不说，也不知自己已卷入王储命案，这使她成为杀手的下一个目标，不过福尔摩斯在千钧一发之际救了她一命，作为报答她只好不情 愿地答应帮助他。
这场调查变得愈来愈诡谲危险，也使得福尔摩斯、华生和希姆横跨欧洲大陆，从英国追踪到法国、德国、最后追踪到瑞士，不过狡猾的莫里亚蒂教授总是早他们一步逃离，同时正意图布下死亡和毁灭的天罗地网，而这只是他更大计谋的一部分，如果他成功，历史将会被改变。
Sherlock Holmes (Robert Downey Jr.) has always been the smartest man in the room...until now. There is a new criminal mastermind at large-Professor Moriarty (Jared Harris)-and not only is he Holmes' intellectual equal, but his capacity for evil, coupled with a complete lack of conscience, may actually give him an advantage over the renowned detective. When the Crown Prince of Austria is found dead, the evidence, as construed by Inspector Lestrade (Eddie Marsan), points to suicide. But Sherlock Holmes deduces that the prince has been the victim of murder-a murder that is only one piece of a larger and much more portentous puzzle, designed by Professor Moriarty. The cunning Moriarty is always one step ahead of Holmes as he spins a web of death and destruction-all part of a greater plan that, if he succeeds, will change the course of history. -- (C) Warner Bros
PG-13, 2 hr. 8 min.
