Stressed and overworked psychiatrist Rose Cotter meets with Laura Weaver, who had several days earlier witnessed her art history professor commit suicide. Laura claims that an entity taking the form of smiling people is stalking and telling her she is going to die. Soon thereafter, Laura begins screaming hysterically and has a seizure. After Rose calls for help, she sees Laura standing up and grinning. With a shard of a broken plant pot, Laura cuts open her neck, killing herself.

Rose sees her manic patient Carl grinning and shouting she is going to die. Rose calls for nurses to restrain him, only to see he was asleep the whole time. Concerned for Rose's mental well-being, her supervisor Dr. Morgan Desai gives her a week off.

The hallucinations continue, making Rose seem unhinged and dangerous to people around her, including her fiancé Trevor and sister Holly. At her nephew's birthday party, Rose's gift has somehow been replaced by her dead cat, horrifying the children. She sees a party attendant grinning at her and curses, then falls onto a glass table, ending the party in chaos. She visits her former therapist, Dr. Madeline Northcott. Madeline suggests her problems stem from her abusive and mentally ill mother, whose death from an overdose she witnessed as a child.

Upon learning Laura's professor was grinning at her before his death, Rose visits his widow Victoria and learns he was affected after witnessing a woman die by suicide. Rose asks her ex-boyfriend Joel, a police detective, to go through police records. They find several cases where someone witnessed a suicide, then a few days later committing suicide in front of someone else, who continued the pattern.