The Devil Inside is a movie that is based on the true events of Maria and Isabella Rossi, mother and daughter. On October 30, 1989 Maria murdered three people in the midst of an exorcism that was being executed on her. The Catholic Church took an interest in the case and thereafter, she was committed to a Catholic psychiatric hospital in Rome and has been living there since. Isabella was not aware of the murders until her father told her what had happened which was three days before he died.
The movie takes place twenty years after the murders at which point Isabella is making a documentary about exorcisms and she heads off to a school in Rome to investigate her mother's exorcism. While there, she meets two priests, Ben and David, who take her along with them to witness an exorcism that they execute on a young girl. The young girl happens to know Isabella's name which is not possible.
Later, Isabella finally visits her mother in the psychiatric hospital and witnesses first hand her mother's strange behavior. Maria happens to know that her daughter had an abortion even though it is not possible for her to know this and this is taken as a sign of possession. Ben and David decide to analyze Maria to determine if an exorcism is warranted at which time Maria attacks them.
Later, Ben repeatedly examines the audio files and determines that there are four demons present. Meanwhile, David is carrying out a baptism and it appears that he attempts to drown the baby in the holy water before passing out.
Soon thereafter, Ben finds David at home and he appears to be possessed. He places a gun in his mouth while crying and kills himself at which point, Isabella immediately has a seizure. This is when Ben realizes that Isabella is now possessed. Ben and Michael, her cameraman who she had brought along with her to tape the documentary, rush her to the hospital and she attacks a nurse. They both take Isabella out of the hospital in the hopes of getting an exorcism. During the drive, Isabella attempts to strangle Michael and she also breathes into his mouth. It becomes very clear that he is now possessed and so Michael speeds into oncoming traffic straight into another car. At this point, the camera cuts out so that the ending is unresolved.