A few days before Christmas break, a young woman, known simply as girl, portrayed by Emily Blunt, makes the choice to use the campus "Ride Share" program rather than catching a bus back home to Delaware. After noticing a post on a bulletin board, she accepts a ride from a stranger, referred to as guy, played by Ashton Holmes.
When she arrives late for her ride, she finds guy asleep in his car, a newer model sedan. As he pops the trunk to help her with her suitcases, she quickly notices that it's crammed with his belongings. He reveals that he has just been “kicked out of my apartment.” While he warms up the car, she rearranges items in the trunk to fit her suitcase, accidentally pulling out a random bag of his belongings. As they depart, the bag is left behind, an eerie token of their beginning.
As girl engages in a self-absorbed cell phone conversation, guy endures her anti-social tendencies silently until he finally seeks to explain the "rules" of their ride share: they each share half the costs, and it's her job to keep the conversation flowing. Their dialogue is clearly awkward, contrasting two vastly different personalities: the introspective loner and the earnest, square graduate planning for a future in philosophy. During a pause, guy curiously inquires about her glasses, prompting a surprised reaction from her, as she only wears them in the privacy of her dorm.
As they journey down the open highway, girl begins painting her toenails, indifferent to guy's existence. After a couple of hours, she asks him to look out for a gas station so she can take a bathroom break. Once they stop at a filling station, she refuses to leave the car until her nails have dried, ultimately forcing guy to carry her inside against her protests. While he chats with the attendant, girl finds the restroom, only to discover its neglected state. After a messy visit, she realizes the bathroom door is locked, and despite her desperate pounding, no one comes to her aid. Eventually, she manages to escape by breaking the door handle.
Once back outside, she is furious with guy for not helping her, although he claims they didn’t hear her due to their conversation. As they continue on, he seems lost, checking directions and insisting he knows where to go. Ignoring her protests, he veers onto a snow-covered side road known as Route 606. The precarious conditions and his insistence on taking a “shortcut” lead them into danger as their car slips and slides in the snow.
As darkness envelops them, an ominous car approaches at high speed. In a moment of panic, they collide into a snowbank, and a radio report warns of plunging temperatures and an incoming storm. As they struggle against the cold, ghostly apparitions—lost souls and spectral priests—begin to surround their vehicle. Initially mistaking them for mere figments of their imagination, girl soon realizes the chilling reality when she tries to touch one and is burnt in the process.
A deeper connection is unveiled as guy reveals how he orchestrated this ride share, having texted her the suggestion, longing for a special memory with her. Through their conversation, girl learns that the bag she left behind contained food he had bought for the trip, contrasting their growing bond with the haunting presence of the spirits that surround them.
In an unexpected turn, a Pennsylvania Highway Patrol Officer, introduced as Martin Donovan, suddenly appears, yet he seems bewilderingly unaware of their predicament. When he tries to pull girl out of the car, it becomes clear he has malicious intent. Just as she fights to break free, guy intervenes, wielding a tire iron to save her, only to realize too late that the officer is also a spirit, one of the most malevolent they’ve encountered.
As the night drags on, the spirits reveal their tragic fate caused by the rogue cop who haunted this road for decades, leading victims to similar doom. Despite their chilling reality, girl attempts to gather her strength, devising a plan to climb a nearby telephone pole and make a 911 call using guy’s cellphone. While uncertain about her success, she manages to make a call before returning to find her companion has slipped away.
Consumed by despair, she drags him out of the car, desperately seeking help when a tow truck arrives, driven by Ned Bellamy. As he loads guy’s lifeless body, he recounts the grim history of this road—a place where death lurks thanks to the rogue cop. However, their hope is fleeting as the same menacing car that initially drove them off the road returns, taking the tow truck driver with it.
Miraculously, both girl and the tow truck driver survive a harrowing accident. Determined to confront the rogue cop’s lingering menace, the tower driver investigates and discovers the burning remains of the cop's car from the 50s. But as he does, he meets a tragic fate, freezing to death when touched by the spirit of his attacker. In a frantic escape, girl evades the vengeful spirit until guy's ghost reemerges, fending off the rogue cop's wrath once more.
With the spirits now by her side, she finds solace in their support, and as she follows guy into the woods, he apologizes, expressing a wish to have explained everything had they had more time. Together, they navigate through the remnants of the past, including the cabin of the priests who had suffered, ultimately arriving back at the service station where their journey first began.
In the film's poignant conclusion, paramedics arrive to tend to girl, who reflects on the harrowing experience by pulling out a snapshot of guy—her driver, her companion, and perhaps the key to her survival.
