When Margaret Toll is killed in an automobile accident, her 15-year-old son David, who witnessed the crash, becomes obsessed with trying to make sense of her death, watching and rewatching the home videos he made when she was alive.

The movie is "Extreme Close-Up" (Monday at 9 on NBC), and director Peter Horton said it was the hardest thing he's ever done. But the trade magazine "Entertainment Weekly" gave the movie its highest advance rating, an A-plus.

The story was written by Horton's "thirtysomething" colleagues, executive producers Ed Zwick and Marshall Herskovitz. Like that series, it has sensitive camera work and background music that Horton said he particularly likes.

What makes the movie technically challenging is its use of film and video techniques as David, played by newcomer Morgan Weisser, tries to figure out what happened to his mother. Then he begins taping his classmates and teachers with a videocam hidden in his knapsack, a move that boomerangs when his geometry teacher kicks him out of class. His high school counselor, who fails to understand him (she insists he "put this experience behind you" and makes him keep a behavior chart), calls in his father, who agrees with her that he's living through video images and not in real life.

"Technologically and dramatically this film was the most difficult thing I've ever undertaken, trying to juxtapose the film and tape together," said Horton.

"The way it's laid out, there's three worlds: the present-day film and the present-day video and the tape on the past. For example, you'll see Laura {his girl friend, played by Samantha Mathis} and David walking toward us and you immediately cut to David's point of view on the camera.

"Weaving those worlds together was the hardest thing I've done. It's trying to find ways of setting up which world we're in without insulting the audience. Making it clear, making it graceful, and still telling the story. Trying to shoot the video as a home movie, making it look like home video so it doesn't look like professionals have done it. In a home video, the camera is no longer an objective observer, it's a character."

Blair Brown ("Days and Nights of Molly Dodd") plays Margaret Toll and Craig T. Nelson is David's father, Philip, an architect and urban planner who tries to hold his family together in the only way he knows how: structure, discipline, "a system," he tells them. He's grown alienated from his oldest son: "You were hers, Steve was mine and Melissa didn't seem to need anybody. I used to ask your mom how you were doing instead of asking you. Now I guess we've got to take care of each other."

Kimber Shoop plays David's brother Steve, and Ellie Raab is Melissa Toll, his sister. Their squabbling seems typical; they still take family vacations. But gradually, something seems wrong. David's camera catches their cheerful mother in moments of pensiveness, then confusion, hearing voices and sounds.

She tells David, "Dad doesn't love -- he cherishes, he respects, but he only loves the parts he's not fed up with ... I need you to tell me when I'm really out there, when I'm getting cloudy. I know I'm getting weird." He responds: "Mom, you always had to live up to the stupid image of what he expects you to be." Not long afterward, she attempts suicide, and it is her favorite son who finds her.

Nelson, who has worked in stand-up comedy and stars in ABC's "Coach," gives one of his best performances to date as Philip Brown, who tries to understand his troubled artist-wife.

And like David himself, Philip Brown begins to wonder if his son's obsessiveness is a signpost toward similar mental illness. After talking with the counselor, he says, "You're one step away from the hospital," and packs up David's video equipment.

Young Weisser turns in an impressive performance of wide range as David Toll, a teenager trying to work out his problems, establish a new relationship with an understanding classmate, and find new ground with his father.

Weisser, said Horton, has "virtually done nothing. He's sort of a discovery after a lengthy bicoastal casting search. We made him come back about four times. His father's an actor, so he's had some theater background, but he had virtually no film background. He's really quite good. Samantha is another jewel."

Horton, who has the role of long-haired, fun-loving Gary Shepherd in "thirtysomething," does not plan to give up acting, but looks toward directing as his future career. "Like John Huston's," he said.