"I Saw Mommy Killing Santa Claus" is the 839th episode of Casualty and the 16th episode of the 27th series. It was preceded by "The Blame Game" and followed by the special episode "Nurse Factor". The episode was directed by Steve Hughes and written by Stephen McAteer. The episode featured the temporary departure of Dylan Keogh who joined the department in March 2011. He later returned towards the start of series 29 in 2014.
As two parents argue, a car rolls into a freezing lake, trapping inside their children Mark and Callum. Back at the ED, the team attempt to resuscitate baby Mark but eventually new locum Dominic decides they can do no more. When Tom disagrees and takes over, Dylan is torn but eventually steps in to help Tom and between them they bring Mark back. With emotions running high, Dylan and Tom square up to each other outside the ED, much to Sam's horror, and Dylan finally admits to Zoe that he has not got over Sam.