Yuukoku no Moriarty: Niji wo Miru Shoujo

In this light novel based on the Moriarty the Patriot series, William visits a department store in London to get a present for Louis and Albert when it is suddenly attacked by a mysterious armed group! Together with help from a mysterious girl, William tries to get control of the situation. Meanwhile, Sherlock becomes Gregson’s assistant as he investigates a locked-room mystery!
(Source: Shueisha, translated)