Casualty: Under Fire will be set in Afghanistan, uncovering the traumatic events which led up to Sam's arrival at Holby.
Show bosses have promised that the special mini-episode will help to explain "Sam's brusque bedside manner and brave but impulsive decision-making".
Casualty's executive producer Johnathan Young commented: "We're always looking for new ways to add value for our loyal fans, and this special episode gives us a chance to explore the past and behaviour of one of our most colourful characters.
"Sam has made a huge impact on the show since her debut last year and is proving hugely popular with viewers, but it's clear she has a troubled past. This Red Button episode will give us a chance to find out just what secrets she hides."
Under Fire will transport viewers to Camp Bastion in Afghanistan, showing Sam (Charlotte Salt) working as the medic on a MERT helicopter alongside boyfriend Corporal Ian Dean.
When a sudden call leads to Sam treating casualties from a suicide bombing, she is alarmed when one of her patients starts to act in an agitated fashion and reaches for a device in his pocket.
A Casualty statement teased: "Sam [then] makes a split-second decision which has life-changing consequences. How do the events affect Sam's state of mind? Just what is the situation with Corporal Ian Dean? And what will be the impact of the events on Sam's long-term future?"
Casualty: Under Fire will air on the BBC Red Button directly after the regular episode being screened on Saturday, April 14. It will also be available to watch on the BBC iPlayer.
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