After nearly drowning as a child, So-yeon (Park Shin-hye) is rescued by her mom then has a really good nap -- which some might call a coma since it lasted ten years. But everyone treats it like she's just had a long rest. No one's particularly shocked that she's awakened after a decade. She herself, aside from some amnesia -- which some suspect is feigned -- doesn't appear to be lagging behind her peers. She can walk and talk with the best of them and if anything, her embroidery skills have improved. She's just a pretty young thing whose expressionless face can be attributed to years of rest. And consider what a nice surprise she's awakened to. The young boy to whom she'd been betrothed has grown up to be a handsome man (Lee Hyun-kyoon). You could call him the man of her dreams except she's not sleeping anymore. And when she does, she has the worst nightmares.
You see, while So-yeon survived drowning all those years ago, her twin sister Hyo-jin wasn't half so lucky. As per usual in horror films, sibling rivalry continues beyond the grave as dead Hyo-jin sabotages her revived sister's engagement, her relationship with their mom, and her reputation at large with the community. Everyone hates/fears the revivified So-yeon because locals have been dying unexpectedly ever since her resurrection. Naturally, no one is going to finger the corpse as the culprit, especially one hiding behind three feet of filthy black hair. (While scientists claim hair can grow a couple inches after death, K-horror and J-horror flicks alike suggest it grows as much as two feet.)
Be it posthumous hair growth or post-traumatic stress disorder, nobody seriously considers cause and effect in The Evil Twin. In a way, writer-director Kim Ji-hwan is aligning himself with the dimwitted family servant who preens in the handheld mirror gifted to her by her mistress and who likes to hang outside to heat her ass over a bonfire. Looking at your own reflection. Warming your bum. Could anyone ask for a better life? Yes, they could. And they should start by requesting much better horror movies.