It's half a story, first off.
They fight creepy mannequins like an old episode of Doctor Who, for some reason.
The main character is "attached" to Briar Rose, so when they're apart from each other, he "deteriorates" and passes out... but he has clearly spent most of his life not even knowing about the house she's in.
So, he inherits the house, weird stuff is in the house (i.e. mannequins), there's no electricity, & it's falling apart. He has dreams that she's trapped in the house somewhere & he has to rescue her. Some lady is there (secondary love interest) because: plot reasons. She knows a creepy old guy that can solve all their problems, of course. They all get together with a tech dude that is snarky (naturally) and he runs an old text about main guy's family through a translation algorithm. Seems like Briar Rose is a virgin sacrifice for the devil & they have to find & save her.... so, they do.
Spoilers: she's evil. They should have finished translating the text & they would have known. Now, she's gonna take over the world.
End movie.