That your father has a new partner is one thing, that he leaves you alone with her more time than he spends with you at home is another, but that on top of that he abandons her when she is hotter than wine is something unforgivable. It is here that she finally realizes her mistake and wants to make up for it with a way out. The thing is, when she gets ready to finally date your dad, she asks for an honest opinion of what she looks like. You can't take it anymore and tell her how you feel and how beautiful she is. She is moved by your words but warms up much more in herself. And so, what can I tell you, in the end the night takes a drastic change because it turns out that in the end your stepmother's departure will not be your father, but you, and you will enjoy her that night; I don't think your father will be bothered by her being "absent" from a date one day, do you?