The synopsis: A teenaged brother and sister, Becca and Tyler, head to their maternal grandparents' house by train for a week while their mum goes on a cruise with her new boyfriend. Neither of the kids have ever met their grandparents before, and plan to document the whole experience on their video camera. As the week wears on, their grandparents become stranger and stranger, leaving both kids terrified and afraid for their lives.
The twist: In a truly chilling moment of M. Night Shyamalan's found-footage film, the kids are telling their mother via Skype about how concerned they are about their grandparent's behaviour, when their mum happens to see a glimpse of the grandparents in the background. She's like, "Hey, not to freak you kids out or anything, but THOSE AREN'T MY PARENTS." The real grandparents were counsellors at a local psychiatric hospital, and ended up being murdered and impersonated by pair of their elderly patients, who have been with the kids all week. There's then a big, deadly showdown between Becca, Tyler, and the faux-grandparents, involving poopy nappies and some big shards of glass.
The amount of damage it'll do to your psyche: 🔪🔪🔪