It all started when Ron and Hermione used a spell banned by the Ministry of Magic to return Hermione's patent's memories. This was moment when unnatural bond appeared between them. To hide this (and avoid being thrown into Azkaban), they decided on a marriage of convenience. After all, who would be hurt by this?
Ron knows he has no chance with Harry, who is engaged to his sister. Hermione, after what Bellatrix Lestrange did to her, is afraid of being close to anyone.
Except there's one problem. Or maybe even more than one.
To win Hermione's heart, Draco Malfoy is ready for everything. Harry Potter reciprocates Ron's feelings, but doesn't know to admit it to him. And Astoria... who also has feelings to the youngest from Weasley's brothers, doesn't realise that she's everything to Blaise Zabini.