Molly Hooper stumbles upon a girl named Charlotte on the dark alleys of London. Charlotte asks Molly for help as she tries looking for her parents after being kidnapped and drugged shortly after arriving on vacation with her small family. After being thrown away on the city's outskirts, Charlotte finds out that her parents had been held hostage on the same place where she had been drugged. Molly, who wants to help Charlotte, immediately turns to Baker Street for Sherlock's help. Sherlock agrees immediately and accepts the case the first time he lays eyes on her. Mycroft joins in after hearing about the case of the girl. As the investigation goes on, John and the Holmes brothers have developed a kind of fatherly bond with the girl. Past memories begin to haunt Sherlock. Hearts have been broken. Who is this girl? And what connection does she have with the Holmes brothers in the past?