Trevor Gooden (Dean Winters) survives a car accident that apparently kills his wife Kirsty Cotton-Gooden (Ashley Laurence) when their car plunges off a bridge into the river below. Trevor manages to escape with his life, but even though police divers find both car doors open there is no sign of Kirsty.
One month later, Trevor wakes up in a hospital and realizes that his wife is missing, but because of a head injury his memory is uncertain and he cannot distinguish between fantasy and reality. Trevor finds himself the prime suspect in a murder case, and has two homicide detectives on his tail. Many strange events befall him, until the Cenobite Pinhead shows him reality. The reality is that Kirsty is in fact still alive. Trevor learns that in reality he was never a good guy. He cheated on his wife with many other women, and tried to get rid of Kirsty by making her reopen the Lament Configuration. She does, but before being taken, she strikes one last deal with Pinhead: she offers to give him five souls in exchange for hers. She kills three of Trevor's mistresses and one of Trevor's friends, who was conspiring to kill her for her fortune.
Trevor is in shock by the revelation and takes off the covers on the operating table, believing it to be Kirsty. The person on the table is not Kirsty, but is in fact him. He is the fifth soul and this entire time he has been in Hell living in limbo. Trying to rediscover his past and piece his life back together was his punishment for his disloyalty to his wife and his inability to accept who he truly was. It seems that she has pinned all of the murders on Trevor and shot Trevor through the head, leading him to crash the car into the river. The film ends with Kirsty walking away from the crime scene with the Lament Configuration in hand.

Question: What caused Trevor to crash the car into the river?
Answer: Kirsty shot Trevor in the head.
Alternative answer: He was shot in the head.