The three-part series, written by Nick Leather and directed by Amy Neil, tells the story of emergency call handler Gabe, played by De Caestecker.
While working for the Strathclyde Ambulance Service in Glasgow, Gabe's world is "turned upside down when he receives a desperate life-and-death call from a distressed woman who appears to know him", the BBC said.
"With Gabe under pressure to work out who she is, he makes a decision that threatens to have devastating consequences."
The BBC One series also stars Sharon Rooney, Daniel Portman, Taj Atwal and Stuart Bowman.
The programme, set in Glasgow, was filmed in Scotland and will air later this year on BBC One and BBC iPlayer.