Rose Weasley never quite fit in. True, she was loyal, brave, courageous, intelligent, but she had inherited her uncle's sense of mischeif, and trouble and chaos often dogged her footsteps. Still, she was expecting to be a Gryffindor. Like her parents. So imagine her surprise when her turn at the Hat came up and instead of Gryffindor, or even Hufflepuff, or Ravenclaw or (god forbid) Syltehrin, the Hat bellows out "Stormbringer!".
As it turns out, there is, or used to be, a fifth house as there was a fifth founder, an emissary from Merlin himself. Some sort of realative, supposedly. Unfortunutly, he left following a fall out with the other founders and the death of his female companion, the house stricken from the records, and his students were asorbed into the other houses, though they never quite fit in with their fellows. Eventually, Stormbringer House passed out of living memory, appeariing only in the earliest of early Hogwarts records. The Hat was instructed to choose students based on qaulities which only fit the other Four. But Rose, apparently, is too much like the Stormbringer to fit into any of the houses.
The dorm remains though, the house elves still keeping it clean and neat over the centuries, awaiting the day when it might be filled with the sound of children once more.
Options are discussed, ideas are tossed around. Hermione is unhappy, mostly because there was something she didn't know.
As the adults argue, Rose encounters the Stormbringer House Ghost. Her name is Dorthy, a young woman with blond hair dressed in clothes that are not from a thousand years ago and a cannister hanging on her belt marked "Nitro-9".
As it turns out, what killed Dorthy is returning, there is no way to contact the Stormbringer, and that leaves Rose, her cousin Albus, and Professor John Smith, the visiting Arithmantcy professor, to do something about it. Strange chap, Smith . . .