It's been three years since the war.
Hermione got a job being an undercover agent. Harry became an Auror. So did Ron. Ginny is head of the Games and Sports department so she and Harry could start a family. Ron tried to control Hermione and ended up in slapping her so she broke it off their relationship.
Draco Malfoy left the country after he, his mother and his farther were proven innocent. Bellatrix Lestrange had Lucius under the Imperius curse and him being head of the house forced Draco into being a Death Eater. He threatened Narcissa to aleast help in bringing the Dark Lords Reign. He hasn't been seen since.
One day Hermione has to work with the three most annoying Slytherins on an important case she cannot back out of. Whilst working with the three she falls in love but with whom.