Amelia Vanek is a troubled and exhausted widow who has brought up her six-year-old son Samuel alone. Her late husband, Oskar, was killed in a car accident that occurred as he drove Amelia to the hospital during her labour. Sam begins displaying erratic behaviour: he becomes an insomniac and is preoccupied with an imaginary monster, against which he has built weapons to fight. Amelia is forced to pick up her son from school after Sam brings one of the weapons there. One night, Sam asks his mother to read a pop-up storybook called Mister Babadook. It describes the titular monster, the Babadook, a tall pale-faced humanoid in a top hat with taloned fingers who torments its victims after they become aware of its existence. Amelia is disturbed by the book and its mysterious appearance, while Sam becomes convinced that the Babadook is real. Sam's persistence about the Babadook leads Amelia to often have sleepless nights as she tries to comfort him.