While the mental divisions of those with dissociative identity disorder have long fascinated and eluded science, it is believed that some can also manifest unique physical attributes for each personality, a cognitive and physiological prism within a single being. Though Kevin (James McAvoy) has evidenced 23 personalities to his trusted psychiatrist, Dr. Fletcher (Betty Buckley), 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 (Anya Taylor-Joy, The Witch), 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.