Graham Young (Hugh O'Conor) is a teenage misfit living in suburban London in the 1960s. He hates his stepmother (Ruth Sheen) but loves chemistry, and the two impulses unite in a wicked plot to slowly poison her. After she dies, he's found guilty and sent to a psychiatric hospital, where an idealistic doctor (Antony Sher) thinks he can be cured. Upon release, Graham finds a job, but struggles to fight back his old urges for murder. The film is based on the true story of the "Teacup Murderer."