Before you write a script or even a plot, you have a “conceit,” and before you can make your movie, you have a “pitch.” And it’d better be a killer.

That’s all “It’s a Wonderful Knife” is — a conceit and a pitch without much of a movie as a result.

It a slasher thriller re-imagining of “It’s a Wonderful Life,” a clever-enough conceit. Let’s see what life would be like if the girl who saved her family and her town from a Nut with a Knife “had never been born.”

That idea was enough to attract two “names” to the cast — Justin Long and Joel McHale. And that pitch, with them attached, got this half-baked project before the cameras and before it was fully baked into something that might be worth watching.

Jane Widdup of “Yellowjackets” plays winsome Winnie, perky Angel Falls High School junior with a boyfriend, a passion for photography, loving parents (McHale and Erin Boyes) and a gay star quarterback brother, Jimmy (Aiden Howard).

Dad works for the local rich guy/mayor (Long) who is gambling it all on a big development he just HAS to pull off. That’s how an old homeowner and holdout winds up dead, along with his granddaughter, Winnie’s bestie Carol (Hanna Huggins).

But plucky Winnie saves herself, her brother and the city from the white masked and cowled angel dolled up like the ornament on the top of the downtown Christmas tree. She kills and unmasks the mayor himself, a guy willing to murder to save his business.

One year later, everybody’s “moved on,” and that’s a tad maddening. Nobody, especially Winnie’s FAMILY, remembers and appreciates her Jamie Lee acts that saved the place? As she stares out into the Northern Lights, she wishes she’d never been born.

Just like that, she gets her wish. I love the “explanation” for this bit of supernaturalism. The lights are “a spirit of someone who died violently.”

They didn’t cover that in my Welcome to Alaska, Thanks for Moving to North Dakota brochures.

The Hell that Angel Falls has turned into is more surreal than realistic. That “Angel” is still going around knifing people, something the mayor’s dopey brother (Sean Depner), now the police chief, seems unconcerned about.

The mayor’s development went forward. And that alternate-timeline party Winnie shows up for is drugged-up and out of control. Nobody recognizes her, not even the town teen “weirdo” Bernie (Jess McLeod) she rashly dissed at the same sort of party one year ago.

But friendless Bernie might believe her, might buy in to her fantastical story, and could have some ideas about how to set this all right.

Director Tyler MacIntyre makes decent use of some wintry Vancouver locations and the script works in a bit of old downtown cinema into the action — a place to girl-bond and battle the bad guy.

But the narrative’s bend from reality to surreal is abrupt and head-scratching, a great leap beyond other “It’s a Wonderful Life” parodies, like the one in “Back to the Future II.”

Treating the entire enterprise as a blood-spattered goof doesn’t pay off, with only Long reading the screenplay that way. His choice to play this mayor as a gladhanding fraud with fake teeth and Dustin Hoffman’s affected voice from “Tootsie” is more curious than amusing.

The slashings are reasonably alarming even as the deaths are perfunctory. The murderers seem more or less properly motivated by the switches in the plot from one “life” to the next. But Winnie’s timeline-to-timeline “change” has an abrupt and weird lifestyle leap in it. The picture never shakes the feeling that this is all a hallucination or dream, one that looks like a made-for-TV movie.

“Yellowjackets” fanatics and horror devotees will excuse a lot, but “It’s a Wonderful Knife” doesn’t just slash through our expectations about what we’re going to see. It stumbles in managing the basics, starting with “Must make some kind of sense, surreal or otherwise.

Rating: bloody violence, drug use and profanity

Cast: Jane Widdup, Jess McLeod, Aiden Howard, Hanna Huggins, Erin Boyes, Katharine Isabelle, Justin Long and Joel McHale.

Credits: Directed by Tyler MacIntyre, scripted by Michael Kennedy. An RLJE/Shudder release.

Running time: 1:27