"Zero Sum Game" is the 820th episode of Casualty and the 39th episode of the 26th series. It was preceded by "All in a Day's Nightmare - Part Two" and followed by "Do the Right Thing". The episode was directed by Graeme Harper and written by Kelly Jones. It is the only episode of Casualty so far that has a title that begins with "Z".
Sam questions whether she is ready for active service when she falters during a training exercise, and her return to Afghanistan is jeopardised further when an army colleague asks her to lie for him. Meanwhile, new nurse Fletch (Waterloo Road's Alex Walkinshaw) makes a favourable impression on everyone except Lloyd, but the pair are forced to work together to help a woman whose initial joy at being told she is pregnant turns to horror when the true diagnosis is made.