CONTAINS BIG, BIG SPOILERS. POSSIBLY.
And yet - which film would it be spoiling? Because there appear to be at least three, possibly four films going on at once, which just leads to confusion. If the film can't make up its mind what the hell's going on, how am I supposed to? I think it's fair that whatever story it thinks it's telling, it's certainly a horror film, but it wants to cover all the genre's bases and ends up as a random sprinkling of various ideas that don't come together.
The basic thrust of Psychosis is that in 1992 a group of anti-motorway protesters are brutally slaughtered by a mad killer. Sixteen year later, novelist Charisma Carpenter has escaped her troubled past and relocated from California to a sprawling country manor somewhere in England with her loving husband, and it's not long before spooky and unexplained things start happening. The mysterious and creepy gamekeeper exposes himself to her; there seem to be intruders in the house, a football-playing boy appears and disappears, the bath runs red.... But her loving husband isn't what he seems and appears to be plotting her mental relapse while spending the nights at orgies. Or is she just imagining everything?
So it starts out as a slasher movie, becomes a home invasion thriller, then a ghost story and a drive-the-wealthy-wife-mad movie. Or a film about a woman actually going insane. But which? It can't be all of them. If she is going mad of her own accord, why get the gamekeeper to drug her food? If the place isn't haunted by the ghosts of the 1992 protesters, why does she dream of them when they're not mentioned in the entire film after that opening scene? But it can't be haunted because the local vicar brings a medium round to the house and she can't find anything. Then, why does the weird gardening girl deny all knowledge of the football boy when in the end it turns out they do know each other? How did the guy painting the windows get inside the house when they'd changed the locks? And then at the end there's a maniac escaped from the local asylum. Is is actually her? Can't be, because she's seen still in her cell - in which case it's a massive coincidence that that very same day, another patient escapes and heads for the very same house and kills off exactly the right people.
Make up your mind what film you're supposed to be making and stick to it. The writing side of it simply hasn't been thought through and the end result is that none of it works. Matters aren't helped, incidentally, by some staggeringly bad acting from the husband. I'm not much of a judge of acting but even to my eye, relatively untutored in the thespian arts, this wasn't even of first read-through standard. Overall, as a movie, it's simply not good enough, even for completists.