The buzz about “Splice” has been almost universally positive. Manohla Dargis of the New York Times described it as “a lot of unnerving fun,” and David Edelstein, on Fresh Air, lauded it as “a strange and wonderful brew!” I agree wholeheartedly with these assessments…but only as they apply to the first half of the film.

Let me explain. The film starts out entirely in a good direction; we’re introduced to Clive (Adrien Brody) and Elsa (Sarah Polley), glamorous scientists working on the bleeding edge—literally—of biology (already we’re surrounded by horror movie lore: Clive is a reference to Colin Clive, the actor who played Dr. Frankenstein in the 1931 classic, and Elsa refers to Elsa Lanchester, the actress who wore the wig as the titular Bride of Frankenstein.) They’re hard at work synthesizing new life forms, the beginning of the film delves straight into all sorts of squishy, uncomfortable subjects like abortion, infertility treatment and the grey area between a “bunch of cells” and an actual being. The dialogue may be a little stilted but there’s just the right mixture of tension and introspection to make things interesting. So far, so good.

Inevitably the two scientists experience a fall from grace, which in this case is brought on by their ill-advised decision to mix human DNA in with one of their experiments. The resulting creature, Dren (“nerd” backwards) comes into the world violently, injuring Elsa in the process, a sign of things to come. As Dren evolves from being tiny and rodent-like to being obviously part-human, it is clear that Elsa harbors an affection for her that goes way beyond scientific interest. By explicitly linking the feminine with the biologically transgressive, “Splice” harkens back to some of the great horror/sci-fi films: “Demon Seed” (1977) and “The Brood” (1979) are the most obvious examples, but the film alludes to everything from the “Frankenstein” series to the “Alien” films to another Cronenberg masterpiece, “The Fly” (1986).

Unlike Cronenberg’s films, which stayed true to their cringe-inducing purposes to the bitter end (and became legendary in the process), “Splice” makes a violent turn about halfway through, refocusing on Dren as a proxy child at the center of an Oedipal merry-go-round and abandoning its initial concern with much more interesting subjects, like, you know, the nature of being human. Instead of exploring the social consequences of creating a semi-human hybrid, the film devolves into a bizarre, futuristic soap opera: Clive becomes attracted to Dren, and Elsa’s dormant maternal instincts reveal themselves to be alternately tender and so cruel that she seems like a transplant from a Brian De Palma film.

In fact, De Palma is an apt figure to invoke here. I’ll spare you the details of how everything ends—my theater laughed through the last half hour, so you can bet it wasn’t good—but suffice it to say that the moral of the story turns out to be yet another cautionary tale about the dangers of women in general and motherhood in particular. Think “Carrie” meets “Alien,” but…bad. Director Vincenzo Natali (“Cube”) and executive producer Guillermo Del Toro had all the elements in place to make the next great horror/sci-fi film, or at least the next passable one. Instead “Splice” ends up being its own special hybrid: half great, half awful.