"Blinded by the Light" has nothing to do with the adverse effects of watching too much television. The CBS movie, at 9 tonight on Channel 9, is a commendable, arresting grapple with the tough problem of religious cults and their spell over the vulnerable young.

Kristy McNichol, 17, and her dull brother James, 18, play siblings split by the cult he has run off to join. "David was my best friend," she says sadly, holding a compass from happier times. Friends of the family who also lost their kid to a cult have him kidnaped and deprogrammed, and so the same plot is hatched for David.

But as Gail Bowers, his sister, McNichol is made the voice of youthful skepticism. "Who's to say he's wrong?" she asks, and, "I don't know what to believe in any more," which is part of the film's cursory but effective inquiry into why young people are attracted to these groups in the first place.She decides to look into the cult on her own and risks getting brainwashed bonkers herself.

The movie opens with a grippingly suspenseful sequence in which the first youngster is kidnaped from the cult by deprogrammers equipped with walkie-talkies and a van; so parent-child relationships have come to this, have they? Yes, they have. Later they hold the youth in a room and shout at him in an effort to break the spell. "He's trancing out," they say at one point, meaning he's hypnotizing himself into an isolated stupor.

Sister Gail is made almost too openminded and proves too long-winded with the lectures as well. The writers are so careful not to make this a thriller about good and evil that they pass around guilt like crazy, and the parents get a drubbing that doesn't make a great deal of dramatic sense.

But, as directed (and photographed) by John Alonzo, this is a tight, engrossing film about a timely, troubling social dilemma.The cast includes Anne Jackson and Michael McGuire as the parents and Keith Andes as the cult leader. Kristy McNichol is unfailingly vibrant as always.

"Nobody ever makes a decision to join a cult," says one of the characters. "They just delay the decision to leave." That line of dialogue comes from life and was contributed by one of the technical advisers hired by executive producer Philip Mandelker ("The Women's Room") after he threw out the first script for "Light" because it wasn't good enough. Before shooting started, members of the cast and crew spent three days in roundtable discussions with two former cult members who'd become deprogrammers themselves. d

"I think we'll get it in the teeth in terms of controversy here," Mandelker says from Hollywood. He expects the movie to anger not only the cult members and leaders but also parents, since the parents in the film are not exactly left off the hook, and deprogrammers, whose methods are depicted as violent and drastic, if necessary.

"Cults are growing in their power, even though media attention has died down," Mandelker says. "We wanted to show that it's still going on and give some understanding as to why it can happen."

Many people may think of the Rev. Sun Myung Moon and his Unification Church when they hear the word "cult." But Mandelker says, "We took great pains to stay away from that particular" group when the script was written because, he says, the group is known to be fast with a lawsuit. The cult in the film is fictitious and any resemblance to a specific group is purely a slip-up on the part of the legal department.

Nevertheless, Mandelker says the fictitious cult in the film is a composite of traits common to eight different cults that were studied. "One of the things we found is that they all go for the most intelligent people. People of lesser intelligence are given menial jobs and eventually thrown out. pThe cults want the best and the brightest.

"Teen-agers are by definition at some point alienated and looking for answers," Mandelker says. "This is an age when there are no easy answers for any of us about anything. Along comes a man and a movement promising answers and a loving environment, and they're hooked."