Where does my sister end and She begin?

The Netherlands, 1887. When her twin sister becomes distressingly obsessed with a recently-found bog body, Lucy initially believes the doctor’s diagnosis of temporary insanity caused by a fever of the brain. But when Sarah’s behavior becomes ever more disturbing, Lucy is forced to consider the idea that something else is causing the changes in her sister, something old and intelligent and intent on staying, even if that means devouring Sarah.

Lucy desperately tries to unravel the mystery surrounding her sister's obsession with the bog body. Who were they? How did they end up buried in the mud, a stone in their mouth? And what do they want?

When Sarah dies unexpectedly - and returns - the evidence mounts that something is indeed consuming her sister (may, in fact, already have consumed her completely), Lucy must decide what is worse: having a parasitic entity that is also partly Sarah for a sister, or no sister at all.