Though Kevin has evidenced 23 personalities to his trusted psychiatrist, Dr. Fletcher, there remains one still submerged who is set to materialize and dominate all the others. Compelled to abduct three teenage girls led by the willful, observant Casey, Kevin reaches a war for survival among all of those contained within him — as well as everyone around him — as the walls between his compartments shatter apart.
Rachel Watson, an alcoholic who divorced her husband Tom after she caught him cheating on her, takes the train to work daily. She fantasizes about the relationship of her neighbours, Scott and Megan Hipwell, during her commute. That all changes when she witnesses something from the train window and Megan is missing, presumed dead.