Disclaimer: I make absolutely no claim of ownership to anything in this story – it all belongs to JKR; please don't sue me. It wasn't until after I'd written this and read it back that I realised I'd made definite suggestions that this story is set during the trio's fifth year but still had Snape as the potions teacher. For the sake of this story let's just assume that Dumbledore found someone else to teach DADA after Umbridge left and Snape stayed down in his dungeon.
Enjoy!