Louis La Rothière was a French spy living in London who Holmes names as one of the three most effective foreign agents in Britain. He is mentioned in two of Holmes' cases of international significance.
When a piece of inflammatory secret correspondence from a foreign head of state went missing from the home of European Secretary Trelawney Hope, Sherlock Holmes named La Rothière as one of the three most likely recipients of the letter, along with Hugo Oberstein and Eduardo Lucas. However, Watson's announcement Lucas' murder shifts Holmes' attention away from him.
La Rothière remains among Holmes' top three suspects, along with Adolph Meyer and Hugo Oberstein, when top secret military plans are stolen from Woolwich Arsenal, apparently by a young clerk named Cadogan West who is later found dead on the railway tracks. However, Holmes' attentions are soon turned towards Oberstein as being the most likely culprit.