Twilight is stricken with insomnia, causing her to be victim to horrible visions and hallucinations. When she does find herself able to sleep, she is haunted by terrifying nightmares. Meanwhile, all across Equestria, magic users, including the Princesses, are beginning to have trouble harnessing their magical abilities, as it seems that magic itself is "running out"

It's up to Twilight to figure out if these mysterious events are interconnected in any way, and to solve the problem of the diminishing magic before it's too late!