"Fatal Error - Part One" is the 986th episode of Casualty and the 25th episode of the 30th series. It was preceded by "Just Do It" and followed by "Fatal Error - Part Two". The episode was directed by Steve Brett and written by Matt Cooke, Vincent Lund and Mark Catley. It was Lofty's first centric episode since his first appearance in March 2014.
The episode begins with Lofty standing over a bed in the ED with a white sheet over, with a deceased person underneath. The rest of the episode is shown in flashback form to reveal what happened during the day.
Five hours earlier, the team are informed of Charlie and Rita's absence for the day, and that Jacob will be in charge of the nursing team for the day. However, Dylan encourages Lofty to step up and take charge now he's a senior staff nurse, and he does so, replacing Jacob. Being completely understaffed, Lofty calls for assistance from an agency nurse.
Elsewhere in Holby a young couple, Kylie and Gary, are escaping from a man who they just conned by faking a robbery and secretly taking his money. Kylie ends up with a cut to her abdomen and is admitted to the ED. Meanwhile, an older boy named Liam goes to check up on his brother James at his former home, but unbeknownst to him, James had set traps on the stairs for intruders. Liam slipped on some marbles and impaled himself on a toy aeroplane lying on the ground, and was taken to the ED with the paramedics.
Back at the ED agency nurse Diane Stuart arrives in order to help out the rest of the nursing staff. However, she almost administers Kylie with the wrong fluid which caused Dylan to shout at her. Lofty later spoke to her and she apologised saying that she was tired and had recently suffered a separation with her husband. Lofty said he would assign her to minor cases for the rest of the day, and told her he would have to report it to her agency. Meanwhile against Lofty's suggestions, Kylie leaves the ED with Gary despite having an existing heart problem.
Elsewhere, Liam receives treatment for his wound, and it soon transpires that he moved out of the home prior to being admitted. James told Liam in front of the staff that he had been living by himself for the last couple of weeks due to their mother not being around. Later in the day, James flees the ED due to the fear of not being wanted by his mother or brother and climbs atop an ambulance. They manage to get him to climb down, but he falls and ends up back in the ED. He is stable, and his brother tells him he will cancel his trip to India in order to look after him.
Meanwhile, Noel continues his hostility towards Big Mac as a result of his deception. Louise notices that something is wrong, but Mac attributes it to his bad back, but she notices that something more is wrong. He has also turned to painkillers to reduce his stress about the situation.
Back on the streets, Kylie starts showing unusual symptoms and Gary calls for an ambulance. Back at the ED Lofty suggests that she would be better off going up to the heart specialists. Lofty travels with Dylan and Diane in the lift, but Kylie goes into cardiac arrest and Lofty is forced to use the defibrillator. He shocks her, and it isn't successful. He prepares to shock again, but Diane's concentration lapses and she has her hand on the patient when Lofty shocks her. Diane falls to the ground and is rushed into resus when they eventually manage to stop the lift down at the ED.
Despite Lofty and Zoe's attempts to save her, Diane dies. Lofty is left feeling shocked and anxious. It then returns to the present where Lofty is standing at the bedside of a deceased Diane, with Dylan telling him to be careful what he puts in his statement in the following investigation as it could cost him his career.