When Elite Cloudsdale Weather Captain Firefly wakes up in the body of an unknown mare, she discovers she's part of a mission to find the bomber of the Ponyville Express. In an assignment unlike any she's ever known, she learns she's part of a government experiment called the Source Pony, a program that enables her to cross over into another mare's identity in the last hour of her life. With a second, much larger target threatening to kill thousands in Canterlot, Firefly re-lives the incident over and over again, gathering clues each time, until she can solve the mystery of who is behind the bombs and try to prevent the next attack.