Lou Jennings, a high school English teacher in the quaint Illinois town of Amber Hill, is struck down with the flu shortly before Thanksgiving. She spends the next few days in bed, cared for by her attentive husband Derek and their two kids, Ben and Ruth.

Upon returning to school, Lou finds a substitute teacher called Evelyn Sparrow holding the fort - as all good substitutes should do - except now, no-one remembers who Lou is. The students, the teachers, everyone in school. As far as they're concerned, Miss Sparrow has always been the English teacher at Amber Hill High, and they've always adored her.

In the days that follow, the once-stable fabric of Lou's life begins to unravel right before her eyes. Friends forget her, family no longer recognise her. Piece by piece, Evelyn takes everything, stripping Lou of all she once held dear, driving her to the brink of madness.

But Lou isn't alone. One student at Amber Hill High - Jason Rennor - notices something about Miss Sparrow, something that's not as it should be. He starts to suspect she may not even be human. Then another student goes missing, and Jason knows for sure - Miss Sparrow is evil, and it isn't long before he finds himself in her crosshairs, fighting for his very survival.

Maurice Baxter knows exactly what Evelyn is. She's left a devastating mark on his life and he's spent months tracking her all the way to Amber Hill, fuelled by grief and vengeance, guided by his dreams. Those dreams bring him to Jason, and then to Lou. Together, they concoct a last-ditch plan to rid the world of Evelyn Sparrow and restore Lou's life before it's lost forever, or the Substitute finds them first.