“Castle in the Ground” is the simplest of rescue parables.

A young man devotes himself to saving his dying mother. And when that fails, this teen in shining armor starts using mom’s leftover painkillers. That’s what changes his focus to the junkie across the hall. Can she be saved?

There’s a little more to writer-director Joey Klein’s Canadian drama, much of it pro forma. But Alex Wolff, Neve Campbell and the case study that Imogen Poots lays out for us make this coming-of-age plunge into the abyss of addiction well worth our while.

Henry’s life is on hold when we meet him. He (Wolff) is crushing up pills and mixing them in jam for his mother (Campbell). He’s trekking to the drug store to get her more pills, taking her to visit her doctors.

She’s still mothering him — “SEAT belt!” But she’s fretting over what he’s missing. His girlfriend is headed off to college. Has he been applying?

“You get better, then I go to school.”

He’s Jewish, and her illness has him desperately diving into prayer. She needs her pain meds and her rest. Damn that noisy neighbor across the hall, the racket he hears, the goings-on he spies through the peep hole. He asks a guy waiting for her to let him in to “turn her music down.”

He sees her bickering for a methadone refill at the pharmacy. She (Poots) is a junky. But even junkies can be reasonable, right? He asks her to keep it down. She happily agrees. “Can I use your phone?” “Give me a lift?” Just this little detour? Lend me $40? $20?

“You owe me, BIG time,” she grins.

Anna is older and cannot be bothered learning his name. And her phone calls are a string of lifelines, cursing out “friends” who won’t pick up, begging her mother for cash. But as Henry’s mother relapses and dies, Henry’s grief takes on forms Anna, in her sentient moments, should recognize.

He’s dazed. “Are you high?” Pause a beat. “Have any left?”

And he’s ignoring her one edict. “Don’t SNOOP.” Henry does. “Wait in the car” becomes “Let’s see what’s taking her so long.”

“How do you know these people?”

“I don’t.”

It’s a shooting gallery. She’s got to “get well,” even though she brags on the phone about “67 days without a poke” (injection). She’s ingesting in other ways. She’s using him. Her occasional words of comfort about his lost mother don’t atone for that.

And others in her circle Henry runs into are more blunt in their warnings.

“She will sell your soul for something THIS (pill-sized) small!”

Actor turned writer-director Joey Klein (“The Other Half”) leaves our hero with no counter-force to help him resist the gravity that pulls him into Anna’s decaying orbit. The religion he’s plunged into is abandoned (he walks out on his mother’s shiva), the girlfriend he pushes away, are no comfort. Nobody is going to rescue Henry.

We can see the perils, why can’t he? There’s no sexual component to this connection. He gives Anna his mother’s phone, and guess who starts to see her as?

Wolff, of “Hereditary” and “Jumanji,” is so screen-seasoned that it’s tough for him to sell “naive and vulnerable” the way he used to.

Campbell gets across the quiet struggle of knowing one’s fate and trying to keep it from breaking her son’s future — concealing, then revealing, edging up to “the talk.”

But Poots is the driving force of “Castle in the Ground,” magnetic, irresistible and insatiable. How deep will Anna draw Henry in? Poots lets us see this as reflexive behavior, myopic and self-interested. We don’t see the wheels turn that generate this performance.

The path Klein sends these characters down is too familiar for “Castle in the Ground” to offer much in the way of twists. But the players take us into this world with them, make us face the same choices and dare us to make different ones than they do.

Put in the same spot, how many of us would?

MPAA: unrated, drug abuse, violence, profanity

Cast: Imogen Poots, Alex Wolff, Neve Campbell and Keir Gilchrist

Credits: Written and directed by Joey Klein. A Gravitas Ventures release.

Running time: 1:45