Twilight Sparkle is called away to tend to Luna when she falls ill. As Luna grows sicker and Twilight more determined to help the younger princess, she begins to uncover dark secrets about the elements, the alicorns, and her own past.
A strange condition has Twilight courting death in the wake of her battle with Tirek. Now her life depends on Luna, and both lives will be affected. Hopefully for the best.