An older man grooms a teenage girl in this disturbing but vital film

Palm Trees and Power Lines begins in the middle of a lazy summer for 17-year-old Lea, played by a remarkable newcomer named Lily McInerny. She lives in a dull stretch of Southern California suburbia with a somewhat scattered single mom — a likable Gretchen Mol — whom she treats with indifference at best and contempt at worst.

Lea spends a lot of her time sunbathing, avoiding her summer homework, scrolling on her phone and hanging out with her friends. While she goes along with a lot of their goofball antics — she smokes and drinks with them, and has a rather perfunctory hook-up with one of them in his backseat — she also seems a little smarter, more sensitive and observant than they are.

One night at a diner, her friends decide to skip out on the check, and Lea, the only one with enough of a conscience to protest, is left holding the bag. But then a man named Tom, played by Jonathan Tucker, seems to come to her rescue and offers her a ride home in his truck. Tom is friendly, assertive and good-looking; he's also 34 years old, and it's immediately clear, from his flirtation with her, that he's a creep.

On some level, Lea seems to understand this even as she and Tom start seeing each other. She doesn't tell her mom or her friends about him, and she clearly knows that the relationship is wrong — but that's exactly what makes it so exciting. She's enormously flattered by Tom's attention, and he seems to offer her an escape from her humdrum reality.

Palm Trees and Power Lines marks a confident new filmmaking voice in the director Jamie Dack, who adapted the film from her 2018 short of the same title with her co-screenwriter, Audrey Findlay. They've written a disturbing cautionary tale about grooming and trafficking. That sounds grim, and it is, but the movie is also quietly gripping and faultlessly acted, and scrupulous in its refusal to sensationalize.

The full extent of Tom's agenda becomes clear when he takes Lea back to his place one night, and it turns out to be a rundown motel room. By that point, you'll be screaming at Lea to make a run for it, but she's already in his psychological grip. The movie captures just how swiftly yet methodically Tom creates a sense of dependency — how he lavishes Lea with attention, compliments and gifts, and gradually walls her off from her mom and her friends.

Tucker, who's been acting in movies and TV shows for years, gives a chilling, meticulously calibrated performance; you never fall under Tom's spell, but you can see how an impressionable teenager might. And McInerny, in her feature debut, shows us the depths of Lea's confusion, the way her desperation for Tom's affection and approval overpowers her better judgment.

In scene after scene, Dack ratchets up the queasy intimacy between the two characters, but she also subtly undercuts it, sometimes by shooting the actors side-by-side, giving their conversations a faintly transactional air. Through it all, the director refuses to exploit or objectify her protagonist. Even the movie's most terrifying violation is filmed with great restraint, which ultimately makes it all the harder to watch.

Dack regards Lea with enormous sympathy, but also with a certain case-study detachment; she never offers the character a way out. There were times when I wished the movie were less unsparing and more optimistic about Lea's future, but its pessimism rings awfully true. While Palm Trees and Power Lines is a story of abuse, it also captures a deeper malaise, a sense of aimlessness and loneliness that I imagine a lot of people Lea's age will identify with. It's a despairing movie, and a vital one.