Two friends, Daniel and Lucas, are driving down a dark road at night. While stopped on the side of the road for Lucas to urinate, Daniel is suddenly killed by an unseen attacker and Lucas flees.
Elsewhere, Maddie and Tyler, Jacob Scipio are celebrating six weeks of being on the road, having recently left their jobs and embraced the van life lifestyle. They pull over on a residential street to camp for the night, where Tyler proposes to Maddie, who accepts. After a neighborhood watch member tells them they're not permitted to camp on the street, they hit the road again.
While driving on a remote highway, Lucas speeds ahead of their van frantically. Moments later, Maddie and Tyler pull over when they see Lucas’s car crashed against a tree. Lucas tries to crawl out of the passenger seat, but is pulled back inside by an unseen force and killed.
In the morning, the couple sees that their van has three claw marks, similar to the ones on Lucas’s car. At a gathering of other van-lifers, Maddie goes through the tents and sees pictures of missing people, as well as a stick figure with three lines going through its body. She meets a woman named Diana Melissa Leo and mentions stopping for a car accident the night before. Diana’s tone becomes ominous, warning Maddie not to stop on the road at night. Later that night, Maddie sees the mysterious figure.
Maddie looks at the dashcam footage of Lucas’s car before the crash and sees the figure sitting next to him in the car. The figure enters the van and tries to strangle Maddie with her seatbelt, but she burns its hand with a St. Christopher medallion. Maddie rushes out of the van and tells Tyler, but the figure has vanished.
Maddie learns there have been other incidents of travelers going missing after their vehicles have been seen with the claw marks. The figure, known as the “Passenger,” haunts and kills travelers. During a screening of Roman Holiday, the Passenger attacks Tyler and Maddie, forcing them to flee. Maddie suggests to Tyler that they get to Flagstaff to find Diana, as she may know how to get rid of the Passenger. During the night, they narrowly escape another attack.
The couple buy medallions to hang around the van as they make it to Flagstaff, asking for Diana. She tells Maddie and Tyler that there was a rumor of a traveler haunted by the Passenger, but sought refuge in the Church of St. Christopher, which is not on any map. The Passenger kills Diana, forcing the couple to flee. While fleeing they try to drive in the daytime, but Maddie falls asleep and when she awakens, it is dark.
While changing a flat tire, the Passenger appears, takes Maddie and crushes Tyler’s leg under the van. After changing the tire, Tyler drives off to find Maddie. The Passenger takes the form of Tyler to trick Maddie, but is thwarted by the lights of the van as Tyler finds Maddie in shock in the scene of another accident. Maddie and Tyler follow a symbol on a billboard, avoiding tricks from the Passenger. Tyler is pulled out of the van and Maddie keeps driving until the Passenger is flung through the windshield. Maddie rams the van into the Passenger, impaling it on a statue and killing it. Tyler reunites with Maddie as emergency vehicles arrive.
