Wikipedia refers to this Korean film as a "mystery action film." I'm calling it "Science Fiction" because the science presented later in the film isn't something we have available yet - despite this being set in the present day. The opening is violent and largely unexplained, with a young girl escaping some sort of research facility and people dying. The facility minder assumes she'll die, but she manages to survive and is adopted by a farmer (formerly an architect?) and his wife. The movie jumps forward ten years to when the young woman (now played by Kim Da-mi) is in high school, leading an ordinary life. Things change though when she auditions (successfully) for a national talent show. This brings unwanted attention in the form of two different sets of threatening people in big black cars. The movie gets staggeringly violent for a while (that's a bit of a spoiler, but if you're not aware of what you're getting into I think the warning is justified).
The movie is well put together and not bad. It's clever (unexpected things happen, reasons are revealed), but I didn't really feel like it was intelligent or there was much of a point beyond the cleverness. And at the end we cut to black on the appearance of a new character and a new threat - thus living up to the more common full title, which is "The Witch: Part 1. The Subversion." Yup, there's to be a sequel (dependent, of course, on receipts.) I'm indifferent: well-constructed, clever, violent Korean flicks are disturbingly common these days, and I'm less into the entertainment value of bloody violence than I used to be.