This fanfic starts in an alternate universe where the story of Harry Potter and the hunt for pieces of Voldemort's soul went awry, and three years after the Dumbledore's death, nearly everyone is dead and only Harry, Hermione, Neville and Luna are still fighting the good fight.
By this point, the battles and the deaths took their toll. Hermione's a research machine doing her best to keep them all alive. Harry knows it's his destiny to suffer a death-by-Voldemort, so he throws himself into battle with madness and abandon. Neville is a hardened warrior. And Luna is a seer who tells stories.
Then, a magical accident takes place and they're thrown into a much kinder universe, where mostly everyone is alive, except Harry, who was never born because Lily Evans married Remus, and except Neville, who was the Boy-Who-Lived and died at the end of the Triwizard Tournament, and Hermione who got killed by the troll in her first year at Hogwarts, and Ginny who killed herself when she was possessed by Tom Riddle's diary. But Snape is there and he's a happier man, who was never a Death Eater. Dumbledore is still alive. Sirius was never in Azkaban.
And of course, the four hardened warriors clash as badly against this background as you'd expect. It's a fun story, alternating between tragedy and pure crack, but it felt rushed to me, and I couldn't connect with it as much as I'd have hoped.