My Adventure in Jermyn Street

The narrator receives a mysterious envelope in the post containing a single theatre ticket for that evening. There is no note or return address, and he has no idea who could have sent it. When he attends the performance, in the third act a mysterious lady takes the seat next to him and asks him to come with her to see her brother George after the performance. They are whisked away by car to a sinister upper floor apartment in Jermyn Street, where the affair becomes more and more enigmatic.