What story does Hitchhiker tell? To be more exact, I don't understand at all.

The main reason is that Penny experienced a car accident when she was a child, and her parents died. Although Penny narrowly escaped, she left a serious mental trauma. After years of various treatments, Penny's psychologist decided to take Penny back to the scene of the accident, hoping that through this treatment, Penny could regain her courage to face the tragedy and overcome her fear of cars. Two women drove all the way to a secluded mountain road and found a mysterious man hitchhiking on the side of the road. Out of kindness, they let the man get on the bus, but what they didn't expect was that the mysterious man was an extremely cruel bloodthirsty killer. When he got on the bus, their treatment trip turned into a death trip, and Penny met something worse than her years of nightmares. That's about it.