A Case of Identity (The Adventures of Sherlock Holmes, Episode 3)

Against the wishes of her stepfather, Mary Sutherland has become engaged to Hosmer Angel. On the morning of their wedding Hosmer elicits a promise that Mary will remain faithful to him "even if something quite unforeseen" occurs, then mysteriously disappears en route to the church. Holmes deduces that Hosmer was Mary's stepfather in disguise, the charade a bid to keep Mary a spinster and thus maintain access to her inheritance. Holmes does not reveal the truth to Mary because "There is danger for him who taketh the tiger cub, and danger also for whoso snatches a delusion from a woman"; he had already advised her to put the matter behind her, though she responded that Hosmer "shall find me ready when he comes back."