When they were just kids, Tim shot and killed their father following a fateful night where he'd earlier killed their mother and was menacing them.
In the intervening time, Tim has had to undergo a great deal of therapy but is finally ready to rejoin society, dismissing what he said at the time about events as the product of trauma.
Kaylie meanwhile, on the surface having led a more normal life maintains that their father was under a malign supernatural influence and has been planning to get proof of this as a form of closure. She has a very detailed plan.
To make the plan happen Kaylie needs Tim and reminds him of a pact they made that night. No good will come of this.
CW: injury detail
Another off the "Best of Blumhouse" list this Mike Flanagan psychological horror is outstanding.
It looks like it's going to be yet another haunty/possessiony jump scare thing and a few early scenes are straight out of that playbook but it's not.
The malign influence is tricky and deceptive. As the movie proceeds it increasingly cuts back and forth between the present and past with a lot of screen time for the young versions of the characters. They're maybe better than the adults with both Thwaites and Gillan being a bit wooden in places.
Kaylie's plan is very modern ghostbuster with video cameras, timers, schedules, bait, canaries, supplies, logs of events and check-ins from people elsewhere. She has researched their adversary at length and planned this like a sensible person would, contrary to so much on screen horror movie behaviour. If the plan fails there is even an automated failsafe to get them out of their predicament.
As the day goes on past and present collide in really gripping and smart ways.
Highly recommended 9/10. Not gory or even scary apart from in a couple of places but it's the best portrayal of "there is an evil thing and it wants us dead" I've seen. This is because there isn't really a monster you can fight, it must be tricked. If you liked Flanagan's Midnight Mass, this is for you.