The Souls, a parasitic but self-harmonious alien species, has arrived on Equestria. With aims to take over the bodies of the planet’s sentient inhabitants, they send down an initial group of Souls and implant them into host bodies. Among them, a young soul finds herself in a new body with this mission: help assimilate the Equestrian race into her species.

This soul finds that her body's previous inhabitant still lives in her mind, refusing to leave, and her notions of right and wrong will be tested amongst a species whose goodness can rival the Souls.

A crossover between the universes of The Host by Stephanie Meyer and MLP FiM