Where does my sister end and She begin?

"I'm in your blood, and you are in mine..."

The Netherlands, 1887.

Lucy's twin sister Sarah is unwell. She refuses to eat, mumbles nonsensically, and is increasingly obsessed with a centuries-old corpse recently discovered on her husband's grand estate. The doctor has diagnosed her with temporary insanity caused by a fever of the brain. To protect her twin from a terrible fate in a lunatic asylum, Lucy must unravel the mystery surrounding her sister's condition, but it's clear her twin is hiding something. Then again, Lucy is harboring secrets of her own, too.

Then, the worst happens. Sarah's behavior takes a turn for the strange. She becomes angry... and hungry.

Lucy soon comes to suspect that something is trying to possess her beloved sister. Or is it madness? As Sarah changes before her very eyes, Lucy must reckon with the dark, monstrous truth, or risk losing her forever.