Spontaneous starts off as the best teen-centered dark comedy since Heathers, then evolves into something unexpectedly profound. Writer Brian Duffield (Underwater) makes his directorial debut with this adaptation of Aaron Starmer's novel, and what a way to kick off a career behind the camera. Sharply scripted and energetically paced, the movie grabs you from the very first scene, makes you laugh hysterically at times, then leaves you with big ideas to think about when it's over.

Katherine Langford (Knives Out) plays Maya Carlyle, a sassy, spunky teenager. Her life gets rocked when peers at her high school suddenly, inexplicably begin blowing up. No one knows why it's happening, they just all fear they might be next. Charlie Plummer (Words on Bathroom Walls) is Dylan, a classmate spurred by these bodily explosions to finally declare his love for Maya. They commence dating as more and more of their friends become human dynamite. Then the government shows up, quarantines all the seniors, and attempts to develop a drug to prevent further tragedy. Of course, the biggest threat to Maya and Dylan's love affair is that a cure won't be found fast enough.

The first hour of Spontaneous has a lot of big laughs. Both the main characters are teens who use a certain type of disaffected sarcasm as a coping skill. It's a big part of what brings them together. Knowing their peers are dying horrific deaths is too much for them to process, so they joke it off. That shared defense mechanism leads to some hilarious lines of dialogue. I laughed out loud at least a dozen times.

A darkly satiric theme is also funny. Teenagers hate to be different, or to stand out in bad ways. They view it as embarrassing. What could be more humiliating than blowing up and leaving everyone around you covered in your blood? The blasts are symbolic of adolescent social trauma. Spontaneous finds humor in how high schoolers react to the anxiety that they might do something so uncool. (My generation would have called it “grody to the max.” I have no clue what terminology today's kids use to convey that outcome.)

The second hour shifts a bit, exploring the very real sense of loss that the phenomenon brings. Besties, boyfriends, girlfriends, and crushes are all cruelly snatched away. Maya's parents (Piper Perabo and Rob Huebel) become paralyzed with fear that their daughter will be next in line. She dreads the thought, too. Spontaneous starts to delve into the idea that none of us know when our time will be up, so we should make every moment count, cherish every minute with the people in our lives, devote every second to accomplishing our goals. The manner in which the film gets to that idea is both sincere and touching.

Katherine Langford has quietly established herself as one of the best young actresses around. She isn't a household name yet, although I suspect she soon will be. Her performance as Maya is thoroughly winning. In her efforts, we see how Maya's initially bemused attitude toward her exploding peers transforms into genuine, all-encompassing panic. The actress is humorous when she needs to be, yet also never less than substantive. Charlie Plummer matches Langford note-for-note, making Dylan irresistibly charming. The two have superb chemistry together, guaranteeing that we become invested in the relationship between their characters.

Spontaneous is the real deal – a teen movie that's smart, funny, sensitive, and perfectly enjoyable even if your own adolescent years are in the rearview mirror. That it's fundamentally about something just makes it even better.

out of four

Spontaneous is rated R for teen drug and alcohol use, language and bloody images throughout. The running time is 1 hour and 37 minutes.