A confetti-like assemblage of different horror and fantasy components blended together, “The Changeover” borrows a little bit of everything to carve out a unique path for itself, even if it ultimately leads nowhere. And while Y.A. audiences might find the “Twilight”-esque tortured high school romance angle appealing, most will walk away from this one feeling like they got a greatest hits version of three different, better movies.

Set in New Zealand and based on the 1984 novel of the same name, “The Changeover” is the story of sixteen-year-old Laura Chant (Erana James), who juggles her high school duties with her permanent side-hustle as her younger brother’s de facto nanny. Laura’s brother, Jacko (Benji Purchase), is a normal five-year-old, easy to distract and prone to wandering, which keeps his sister busy. This proclivity for disappearances leads to Jacko’s run-in with a local antique dealer, Carmody Braque (Timothy Spall), who stamps the little boy’s hand with ink that does not wash off.

This stamp leads to a dramatic change in Jacko, who starts exhibiting strange behavior on top of suffering from physical ailments that endanger the boy. Laura, always sensitive to premonitions, begins to sense that something supernatural is at work, and that Jacko is in more danger than her mother or the child’s doctor’s realize. Laura’ mystic sensitivity leads her to a boy from school, Sorensen (Nicholas Galitzine), who seems to intuitively understand what’s going on even before Laura lays it all out, and agrees to help her with Jacko, the stamp, and Braque.

Directors Miranda Harcourt and Stuart McKenzie make a choice with the script at this point, electing to give the audience just enough information to get a sense of what’s going on without halting the movie to spell it all out. Sorensen unpacks the realities of this universe to Laura, which is populated by witches and vampire-like succubus creatures, yet the audience is given only fragmentary information on Braque and his connection to Jacko. The basic outlines of unique and manageable mythology are here, yet there’s just enough mystery left in to keep the audience guessing about what’s going to come next.

On the one hand, this demonstrates a faith in the audience that isn’t often found in Y.A. thrillers, yet on the other, it isn’t quite earned, here. For book readers, the mythology of “The Changeover” might make sense, but for someone just taking this movie’s word for it, things come off muddled. Laura’s relationship with Sorensen is barely established before the latter starts unspooling the warlock and witch-rules of this world, which seems odd even before a coven starts recruiting Laura: a move that recalibrates the out-of-nowhere-weirdness levels for the rest of the picture.

And strap in, because stuff only gets stranger as the film proceeds. An “Inception”-meets-“Nightmare on Elm Street” dream-battle occupies much of the third act, where one has to parse out what’s really happening versus what isn’t. Again, there isn’t much of a narrative set-up for these moments, or connected mythology to explain them, and while the script strings them together, it’s easy to get lost in the fog of what is and isn’t happening (and why). Vampire-like rules about inviting someone in are thrown around, as is the talk of mystic sensitivity, yet it’s all tied to other elements of the horror ethos like random, mismatched pieces on a Mr. Potato Head toy.

The actors do a decent job selling most of this, with Erana James bearing the bulk of the story’s weight of contrivance. She transitions with ease between the exasperated teenager and put-upon magic victim, never once grinding her gears. Spall gets a majority of the juicy stuff as the mysterious, menacing Braque, and makes a meal out of every scene he’s in while managing his tone and posture in such a way that never over-sells any of his work. His knowing grin, and affable demeanor sit just above the surface of his character’s façade and make for a magnificent villain.

There are a number of interesting ideas at work here, and had the script taken the time to flesh out this world a bit more while retaining some of the coy mystery of the presentation, “The Changeover” might have clicked more than it does. As it stands, there’s just not enough narrative detail about this world to get all that invested in it, and what does come arrives a bit too late and half-formed. A confusing mind-f*ck of a finale doesn’t help things all that much, nor does a flat romance between Laura and Sorensen that feels like it is sponsored by Vicodin.

An interesting if somewhat incomplete horror thriller with decent performances, “The Changeover” is nevertheless hampered by a script that doesn’t seem to know how to connect all its narrative dots. There’s an outline of a decent movie here, and interesting, familiar ideas at work, they just don’t come together in any kind of satisfying or coherent way. This film might take place in a world lousy with witches and warlocks, but it is desperately short of the magic needed to overcome its flaws. [C]