starstarstar-outlinestar-outline(2 stars)

With a wink and a nudge, “Freaky” begins with the on-screen title “Wednesday the 11th,” signaling that, despite the gruesome violence by a Jason Voorhees-like serial killer in its first five minutes, it’s not a straight slasher film but slasher-adjacent. Meaning that this mildly amusing body-swap horror-comedy (whose title references the supernatural high school comedy “Freaky Friday”) will get underway with all the ingredients of a “Friday the 13th”-style bloodbath — randy adolescents, sex, alcohol, a masked killer out of urban legend and multiple killings by such creative implements as a bottle of Montrachet wine and a toilet seat — before taking a slight left turn.

After this scene-setting prologue, the killer, or Butcher, as he is known (Vince Vaughn), targets the film’s protagonist Millie (Kathryn Newton), a mousy high-schooler. But a mishap involving a magical, ancient Aztec dagger, stolen from the scene of his most recent crime, doesn’t quite kill her, instead causing the two of them to switch bodies. Millie is now a psychotic serial killer, and the Butcher is now a 17-year-old girl.

To be sure, there are some small satisfactions to this scenario, including the puncturing of tired genre conventions and the schadenfreude of watching Millie dispatch her tormentors: a teen mean girl and a trio of leering jocks. But the chief pleasure of the film derives from the incongruity of Vaughn’s performance, which the actor never pushes into caricature.

Still, it’s a giggle, not a guffaw.

The story proceeds as you might expect: with the now-murderous Millie, hampered only by her girlish physique, continuing the Butcher’s deadly spree; and the Butcher, addled by a hormonal crush on a cute classmate (Uriah Shelton) and aided by Millie’s BFFs Josh and Nyla (Misha Osherovich and Celeste O’Connor) — who have to be convinced that their friend is trapped in the body of a hulking assassin — in desperate pursuit. The curse of the Aztec dagger, it seems, can be reversed only if the two parties are brought together by midnight on Friday the 13th and one of them finishes the job he — or should that be she? — started.

The logic and precise mechanism of this little loophole is never made terribly clear. The Butcher, who is now really Millie, is supposed to plunge the dagger into Millie, now the Butcher, and that will cause their souls to trade places again? But wouldn’t that kill Millie?

Don’t think about it too hard. “Freaky” isn’t AP Bio. It’s a shop class project: a couple of mismatched planks cobbled together well enough to get a passing grade.

R. At area theaters. Contains strong bloody horror violence, sexuality and coarse language throughout. 101 minutes.