This is a heavy outline atm but
Narcissa is dead and Draco’s been half in love with Hermione his entire life. Unfortunately before Narcissa died Lucius was a cunt and drew up a contract between his son and Astoria greengrass. Nothing can be done to break it so even when draco comes to him and tells him he's in love with hermione there's nothing he can do. Draco and hermione break up, draco and astoria get married. Draco is miserable. Lucius will do anything to see his son happy again so he approaches draco with an idea. He will marry Granger so that the two of them can be together. All three of them agree and Lucius is happy to see his son happy again, even if it means he is still quite without a wife for all intensive purposes.
Cut to 6 months down the line when he's wandering down the hall and catches Astoria leaning against the library door breathing heavily. He steps up behind her and see his son and hermione fucking right there on one of the tables and realises that astoria is getting off to it. He helps her along and after she comes she flees.