A boy clad in a yellow jacket is seen fleeing through the woods, appearing injured and ultimately collapsing in despair. Meanwhile, five teenagers—Nolan, Bess, Reggie, Queenie, and Karl—embark on a field trip accompanied by their bus driver, Joseph. During the ride, playful banter unfolds as Karl teases Queenie, while Nolan harbors feelings for Bess and Reggie sulks in the backseat.
To keep the teens entertained, Joseph poses a riddle related to a seven-letter virtue, but soon their journey takes a turn when a fallen tree obstructs their path, prompting Joseph to find an alternative route. The situation escalates when Joseph steps out to clear a dead deer, only to return at gunpoint, confronted by the notorious escaped convict, Pedro Minghella, whom Karl recognizes from the news.
Pedro hijacks the bus and orders everyone to discard their phones to ensure they can't call for help. As night falls, the bus mysteriously breaks down inside a tunnel, leading Pedro to hold Queenie hostage while demanding Joseph repair the vehicle. With the lights extinguished, a terrifying shape emerges, revealing itself to be a monstrous creature that fatally attacks Joseph, horrifying the teens who witness the gruesome event.
Struggling for survival, Pedro chooses Karl to retrieve the keys Joseph was carrying, but Nolan insists on going instead. In a bold move, Bess exits the bus to fetch the keys and successfully returns. However, their ordeal intensifies when Pedro forces Reggie to attempt to start the bus under threat, but the vehicle fails to respond. In a panic, Pedro steps outside, only to fall victim to the creature himself.
Desperate to escape, the remaining teens manage to flee but realize they need to rejuvenate the tunnel lights to ward off the creature, which turns out to be sensitive to light. Nolan and Bess venture off to locate the main generator while the others stay together. During their search, they stumble upon evidence indicating someone had previously encountered the creature and attempted to destroy it.
As Queenie asks Reggie to accompany her to relieve herself, she is suddenly captured by the creature. Reggie and Karl intervene, fighting off the beast just in time, though it leaves Queenie with a severe injury. Reggie is also targeted but is subsequently rescued when Nolan and Bess manage to turn the lights back on, prompting the creature to retreat.
Reunited, the group examines the evidence they discovered, linking it to a man named Giulio Sarpi, who lost his sister Isabella to the monster in 1971. His lifelong quest to defeat the creature tragically ended with his demise, leaving his corpse for the teens to find. Realizing they must take action, the friends devise a plan to confront the monster.
Bess and Queenie cleverly lure the creature toward the exit, where, under the daylight, it finds itself cornered by the boys and their torches. As the creature is engulfed in flames, it lets out a haunting scream before collapsing, seemingly defeated.
Emerging into town together, the teens are closer than ever. They reflect on their journey, revealing that they ultimately solved Joseph's riddle—the answer was courage, a trait that cemented their camaraderie and aided their escape. However, their tale isn't quite over; investigators arrive at the tunnel, and as one detective enters with a flashlight, the light unexpectedly flickers, followed by eerie sounds from the lurking creature that leaves the screen fading to black.
