The Memoirs of Sherlock Holmes - The Yellow Face
A man comes calling, Grant Munro, tells a story about his wife Effie Hebron. She had a husband and child in America. They both died of yellow fever.
Grant says he and Effie were super in love. She even signed over all her money to him. One day, she asks him for money and a few days later, new people move into a nearby cottage. The woman is extremely rude to him. He tells his wife and later that night he catches her sneaking out.
Next day, he sees a yellow faced man from an upstairs window. Then his wife walks out of the house. She begs him not to go in and she wouldn't lie to him again. A few days later, he comes home early and catches her there. He goes in but it was empty inside.
Holmes says to go back and telegram when the occupants come back to the house.
The face is actually Effie's child wearing a mask. Turns out her first husband was African and since the child was darker than the dad, she couldn't admit to it still being alive. Grant accepts the child and tells Effie should should have had more faith in him.
Holmes' theory was that the man was the surviving husband and was blackmailing her. Turns out he was wrong. He tells Watson to reference this case whenever Holmes needs to be humbled.