As a general rule, didacticism is never becoming in a movie. This is even more true when the moral lessons in question are as banal as the ones in “Snow Cake,” by the veteran Welsh director Marc Evans.

Screenwriter Angela Pell has an autistic son, so she decided to write a script that would teach us about autism — and about how autism teaches us about life. Unfortunately, the lessons tend to boil down to the following: Live for today and be tolerant of others’ foibles.

Under the circumstances, it is astonishing that anything beginning from such unpromising material can generate any emotional heat at all. That “Snow Cake” does this, a bit, owes to Mr. Evans and his director of photography, Steve Cosens, in cooperation with the wintry landscapes of Ontario, and two fine actors — Alan Rickman and Sigourney Weaver.

Mr. Rickman plays Alex Hughes, an Englishman with a secret sorrow who is driving across Canada to Winnipeg to meet someone of significance in his past. Along the way, he picks up a talkative young hitchhiker named Vivienne (Emily Hampshire), who is soon killed when an 18-wheeler slams into their car.

Unharmed but badly shaken, Alex feels compelled to visit Vivienne’s mother, Linda (Ms. Weaver), in Wawa, on the shores of Lake Superior, to express his regret, even though he wasn’t at fault.

When he arrives at her house, however, he is shocked to find Linda completely unemotional about her loss. Soon he realizes that she is autistic and more or less incapable not only of grief but of “social,” as she disdainfully calls normal human contact.

Since there is no one else to do it, Alex agrees to stay on for a few days and take charge of Vivienne’s funeral, even though this means putting up with Linda’s many bizarre habits — like eating snow or attempting to feed her dog an unpeeled banana — and her obsessive neatness.

It’s not difficult to see where this is going. Alex learns about life and living in the present moment from the autistic lady, who is, of course, unable to do anything else. He also revives his stalled emotional life through a sexual liaison with Maggie (Carrie-Anne Moss), the obliging woman next door — who, so Linda tells him, is a prostitute.

Maggie is a life lesson too far, I’m afraid. Though not a prostitute, she is improbably beautiful and improbably ready to take charge of Alex’s emotional needs. “I really like you,” she tells him over a shrimp cocktail shortly after he arrives at her house for dinner. “And I hate having sex on a full stomach, so can we just skip the main course and move next door?” No wonder Alex takes Linda at her word and offers to pay afterward.

Not only is Maggie too easy, so is the lesson that she has to teach about emotional availability — too easy and way too movie-conventional. The rapidity with which she coaxes Alex out of his hardened shell of grief and guilt into an emotional resurrection only accentuates the film’s preachiness. It also undercuts the good work being done elsewhere with the help of its bleak and painterly views of Lake Superior and its quirky but effective vision of what it means for life to go on.

And the glibness returns at the end, as “Snow Cake” tries to make Linda into a kind of idiot-savant, just as Maggie is a therapeutic prodigy. Alex departs with this encomium to the former: “You have been very annoying. You’ve also been a friend. I’m going to miss you. … You are the only person I have ever met whom I didn’t have to explain or even justify myself to.”

Well, except for Maggie I guess.

That Alex has to learn to forgive himself and others is fair enough, but it doesn’t require this awkward and unconvincing erection of nonjudgmentalism into a principle — as if it were all so much easier for us, now that Alex has spelled out the lesson, than we know it has been for him.

Part of the problem is that we have to accept that the autistic can’t help but be anti-social. But for the same reason, they can’t be a lesson to the rest of us — as, when it overreaches like this, the film wants to make us believe they are.

Reveling in our own compassion and understanding, we are liable to find big moral and spiritual truths where there are, in truth, only much more humble forms of solace to be had. “Snow Cake” should have stuck to these.

jbowman@nysun.com