The story tells of a unicorn, named Harvest Chaser, who after a series of unfortunate events gets banished to the outside world out of his Stable -- Stable 65. The events are taking place in the ruins of a city, called Trotfield, which was once an important military checkpoint, yet is now a large ruin, which the Enclave, the Red-Eye's Raiders and the others want to get their hooves on.

This fanfic is based on the one written by Kkat, called "Fallout: Equestria".
I suggest you reading the original in order to understand the core elements in the story.