Liam just told Steffy that he has chosen Hope the day after their divorce is final. Steffy is devastated and is home alone drinking wine at her apartment. Bill has heard about what Liam did and goes to see her. (she is not drunk). They talk and one thing leads to another and they make love. After both realize it was a mistake and agree not to tell anyone and not to let it happen again. Only it becomes evident they can't hide that one night.
Note: I am not a Hope or Katie fan.