Following a crisis that threatens the future of Sweet Apple Acres and the Apple family as a whole, a thoroughly dejected Applejack runs away from Ponyville, thinking she’s better off dead. But finding her in the night, Rainbow Dash helps Applejack and shows what life in Equestria would be like if she was never born.

Roughly similar to the film "It's a Wonderful Life". :)

Note: The plot above doesn't actually begin until further on into the story. The majority of the story, similar to the film, is the build up of events that have happened throughout Applejack's life that lead up to the main plot.