Cara Harding (Julianne Moore) is a psychologist who specializes in Multiple Personality Disorder and has developed a highly skeptical opinion on whether this disorder actually does exist, or whether it merely masks other factors of a disturbed human mind.
Her theories are heavily put into question when she encounters David (Jonathan Rhys Meyers) who's changing personalities are coherent with those of murder victims to the degree that they know facts about their lives that only the deceased could have known.
Soon people in Cara's life exhibit strange marks and symbols on their skin and die. Symbols, Cara will soon discover, are linked to witchcraft and David's shocking secret will be revealed.

Shelter is a rather peculiar interplay of two distinct genres. It starts off as a tense and intriguing psycho-thriller but soon descends into a quite puzzling twilight between the afore-mentioned and a mystery-/horror movie which does not manage to pursue a coherent line of character-development and all to quickly becomes yet another "crazy, creepy dude chases poor innocent woman through the woods" flick.
The claustrophobic and dark atmosphere, as well as the story-material sometimes (I would imagine unintentionally) draw parallels to Identity (2003), yet it is that atmosphere that enables the viewer to maintain interest in an increasingly accelerating plot that more and more unfolds in a bizarre and unexpected twist.

Moore and Meyers are well-cast for this movie and pull off a decent performance despite a mediocre story and an editor whose skills leave a lot to be desired.
So is it worth spending part of your children's inheritance on it? If you like Identity or movies that do not shy away from touching the outlines of other genres within their story, in this case thriller and mystery/horror, then you probably won't waste your time watching Shelter. It is not a must-see but should do the job of entertaining you for 112 minutes. Also the climax at the end slightly helps to make this movie worth remembering!