Mum is late getting out of bed one morning, so her young son brings in her medicine on a tray, hoping to save her a few minutes in her daily routine. At first she is angry that he has handled her pharmaceuticals. Then she asks if he’s sure he mixed it properly. He has; he’s seen her do it enough times.

Then we realize the “medicine” is heroin, and Mum can’t start the day without it. Oh, and it’s the boy’s birthday, which she has forgotten but which her dealer remembers.

And so begins “Pure,” a grim, somber story about a drug-addled mother and her stoic little boy. It is so well-acted that it deserves admiration, but some of its pedestrian plot elements drag it down toward mediocrity. If you’re going to endure a movie this bleak, it had darn well better be a GREAT movie.

Directed by Gillies MacKinnon (whose “Hideous Kinky” also dealt with neglectful mothers), “Pure” is a cautionary drama on the order of “Requiem for a Dream” and “Trainspotting,” focusing on a little family on the mean streets of London. Mel (Molly Parker) is the addicted mother, single since the death of her husband, though her drug dependency predates that. She has two children, 10-year-old Paul (Harry Eden) and the somewhat younger Lee (Vinnie Hunter), whom the movie has no use for and quickly shuffles off to Grandma’s house.

Paul doesn’t know what a “junkie” is and resents the schoolyard bullies who say his mother is one. His mom’s friend Vicki (Marsha Thomason), she seems more like a junkie, always on the verge of death and unopposed to selling her body in exchange for a fix. The sight of Vicki’s neglected baby girl will break your heart.

Alarmed by Vicki’s latest overdose, Mel wants to kick the habit herself and seeks to do so cold-turkey, on her own, not in a clinic but barricaded in her bedroom. It falls on Paul to supervise her, to keep her locked up and away from the junk no matter what she says. This plan does not sit well with Lenny (David Wenham), Mel’s dealer and sometime lover, who obviously does not want to lose a client to sobriety.

Alison Hume’s screenplay is spot-on in its depiction of addiction, withdrawal and desperation, and Parker’s performance as Mel is harrowing. There is also a good turn by Keira Knightley as a diner waitress, also a customer of Lenny’s, who befriends Paul during the crisis.

Hume goes a little off-track with Lenny, however, his inevitable comeuppance happening at the expense of realism as it succumbs to movie drug-dealer clichés. (The cops are involved.) I would like to have seen all that go down with a bit less sensationalism.

The star in all this is Harry Eden, barely 11 when the movie was filmed, yet utterly adult in his handling of the role. Paul has a tough face, hardened by neglect, distrustful of nearly everything — but he loves his mum. His devotion to her is powerful, and very movingly conveyed by Eden. The movie, flawed and dreary though it is, may be worth seeing just for him.

(A note on marketing: “Pure” premiered at the Toronto Film Festival in 2002, opened in the U.K. the following spring, and is only now being released in the U.S. In the interim, Keira Knightley has become famous (“Pirates of the Caribbean,” “Love Actually”) and so her face is most prominent on the movie’s poster, despite her character being secondary. Harry Eden, on whose young shoulders the entire film rests — well, sadly, the lad has failed to become an international superstar in the past three years, so he is relegated to the background, where he is barely visible.)

B- (1 hr., 36 min.; R, some strong profanity, a lot of drug use, a little violence.)