The film's opening establishes the ruthless nature of the entity when Lucas's friend Daniel is killed, forcing Lucas into the fatal crash that Tyler and Maddie later witness. By stopping to help Lucas, the couple unwittingly accepts "The Passenger" into their lives. The entity acts as a curse passed along desolate highways. As the narrative progresses, the use of onboard van cameras reveals the entity slowly invading their safe space.
In the chaotic second half, the couple seeks the help of Diana (Melissa Leo), who explains the hobo code and the mechanics of the curse. Despite their desperate attempts to outrun the demon and cleanse the van, the climax reveals that the van itself was marked for death from the beginning. The film's abrupt and bleak ending subverts expectations, illustrating that the entity is an inescapable force of nature, turning their symbol of freedom into a moving tomb.