"Shock to the System" is the 1014th episode of Casualty and the 10th episode of the 31st series. It was preceded by "Night of the Loving Dead" and followed by "Thirty Years". The episode was directed by Tracey Larcombe and written by Jon Sen. This episode also sees the arrival of Sebastian Grayling, portrayed by Rik Makarem.
A new F1 doctor, Sebastian, joins the ED on a rotation. On his way in on his bike, he accidentally hits Dylan's dog Dervla and she runs off. His smarmy and cocky attitude instantly clashes with Dylan, who's already decided to mentor him. Dylan soon begins to wonder where Dervla is and contacts Noel for the CCTV footage.
Meanwhile an elderly woman named Supriya is found on the floor by her carer Elena, near a fire caused by a candle. Supriya's daughter Preethi arrives and accompanies her into the ED. Once there, Dylan and Seb begin her treatment, and Supriya makes it clear that she doesn't want to be resuscitated. Dylan is unimpressed with Seb when he reveals some of Supriya's results to Preethi but he assures him that it won't happen again.
Elsewhere, Cal and Alicia arrive for work following a night together. However, when she declines to meet up with him that night he takes it personally and is offended. They begin treating a man named Andrew who has a dislocated shoulder. Andrew says that his career is in fighting for money and although his girlfriend Leanne along with the doctors dissuade it, he continues anyway. When Cal gets him alone, Andrew admits to him that he's been taking pills for a while, but not to tell Leanne the truth.
Cal makes Alicia think about her own love life when he questions how many serious relationships she's been in. He later sees her crying and she tells him that she's just got off the phone to her mum who's upset about her dad. They're interrupted by Leanne who realises that Andrew's gone. They find him getting in a taxi to go to the fighting ring, and Leanne follows him there. Just as he's about to step out the ring, his opponent hits him and he's brought back into the ED. Despite needing resuscitation, Cal and Alicia manage to save him and he's taken upstairs for further observation.
Preethi becomes suspicious of Elena when she sees her wearing one of her mother's rings. Robyn attempts to sort out the situation but ends up coming over faint as a result of the pregnancy. Noel informs Dylan that they don't have CCTV footage but the Holby Pulse's camera was on. He later interrupts to show Dylan the footage which shows Seb hitting Dervla on his bike on the way to work. Although he's angry, this becomes a minor concern when he realises that Seb has resuscitated Supriya. Preethi is relieved that she's back and thanks Seb. She also asks for Elena to be brought back to see Supriya before she dies.
A meeting with Elle doesn't go well and she informs Seb that they'll have to suspend his rotation due to the fact Dylan is no longer willing to mentor him. However, on his way out of the department he hears Dervla and brings her into the staff room to treat her. Upon learning this, Dylan decides to give Seb another chance in the ED. Cal and Alicia plan to meet at the pub after work and they decide to give their relationship a go.