"What Lurks in the Heart" is the 1021st episode of Casualty and the 17th episode of the 31st series. It was preceded by "New Year, New Me, New You" and followed by "Back to School". The episode was directed by Jamie Annett and written by Rachel Smith, and is the first episode to air in 2017. This episode marks the final appearance of Sebastian Grayling, portrayed by Rik Makarem.